diff --git a/.editorconfig b/.editorconfig
index 6bf6372..b8a739d 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -11,3 +11,6 @@ indent_size = 4
 
 [*.md]
 indent_size = 2
+
+[{Makefile,*.mak}]
+indent_style = tab
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 7d7fce5..e7d03e8 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -14,11 +14,11 @@ build-allinone:
 test-run:
   stage: test
   script:
-    - t/run.sh
+    - make test
 
 test-run-allinone:
   stage: test
   dependencies:
     - build-allinone
   script:
-    - SCRIPT=~+/simplify_static_dir.pl t/run.sh
+    - SCRIPT=./simplify_static_dir.pl make test
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..f56e4e0
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,8 @@
+PROVE = prove -rv
+
+TESTS_DIR = t
+
+test:
+	$(PROVE) $(TESTS_DIR)
+
+.PHONY: test
diff --git a/t/TestFunctions.pm b/t/TestFunctions.pm
new file mode 100644
index 0000000..94441ff
--- /dev/null
+++ b/t/TestFunctions.pm
@@ -0,0 +1,130 @@
+package TestFunctions;
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+require Archive::Tar;
+use Cwd qw/
+    abs_path
+    chdir
+    getcwd
+/;
+use File::Basename qw/
+    dirname
+/;
+require File::Temp;
+
+use Exporter;
+our @ISA = qw/ Exporter /;
+our @EXPORT = qw/
+    are_hardlinked
+    file_exists
+    filemtime
+    has_mtime
+    mktempdir
+    prep_tar
+    run_script
+    run_script_capture
+/;
+
+use constant SCRIPT => $ENV{SCRIPT} // abs_path dirname(__FILE__) . '/../simplify_static_dir-main.pl';
+
+sub are_hardlinked {
+    my $starter = shift;
+
+    my $gen_ident = sub {
+        my ($dev, $ino) = stat $_[0];
+        return "$dev:$ino";
+    };
+
+    my $starter_ident = &$gen_ident($starter);
+    foreach my $file (@_) {
+        if (&$gen_ident($file) ne $starter_ident) {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+sub file_exists {
+    foreach my $file (@_) {
+        unless (-e $file) {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+sub filemtime {
+    (stat shift)[9];
+}
+
+sub has_mtime {
+    my $mtime = shift;
+    foreach my $file (@_) {
+        if (&filemtime($file) != $mtime) {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+sub mktempdir {
+    return File::Temp->newdir(
+        TEMPLATE => 'tests.XXXXXX',
+        TMPDIR => 1,
+        CLEANUP => 1,
+    );
+}
+
+sub prep_tar {
+    my $tarball = shift // (dirname(__FILE__) . '/t.tar');
+
+    my $td = &mktempdir;
+
+    # Note: Using chdir from Cwd automatically keeps $ENV{PWD} up-to-date (just
+    # in case)
+    my $oldpwd = &getcwd;
+
+    chdir $td;
+    my $tar = Archive::Tar->new;
+    $tar->read($tarball);
+    $tar->extract();
+    chdir $oldpwd;
+
+    return $td;
+}
+
+sub run_script_capture {
+    my @cmd =(SCRIPT, @_);
+
+    use IPC::Open3 qw/ open3 /;
+    my $stderr = File::Temp->new(
+        TMPDIR => 1,
+        CLEANUP => 1,
+    );
+    my $stdout = File::Temp->new(
+        TMPDIR => 1,
+        CLEANUP => 1,
+    );
+    my $in = '';
+    local *CATCHOUT = $stdout;
+    local *CATCHERR = $stderr;
+    print STDERR "+ @cmd\n";
+    my $pid = open3 $in, '>&CATCHOUT', '>&CATCHERR', @cmd;
+    waitpid $pid, 0;
+    seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
+
+    return (
+        $?,
+        (join "\n", <CATCHOUT>),
+        (join "\n", <CATCHERR>)
+    );
+}
+
+sub run_script {
+    print STDERR '+ ' . SCRIPT . " @_\n";
+    system SCRIPT, @_;
+}
+
+1;
diff --git a/t/freed-bytes-commas.t b/t/freed-bytes-commas.t
new file mode 100644
index 0000000..1598500
--- /dev/null
+++ b/t/freed-bytes-commas.t
@@ -0,0 +1,31 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 1;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $test_dir = &mktempdir;
+&put_file(
+    "$test_dir/1",
+    "$test_dir/2",
+);
+
+my (undef, $stdout, $stderr) = &run_script_capture('-f', $test_dir);
+ok "freed 1,048,576 bytes (1 MB)\n" eq $stderr, 'prints freed bytes with commas';
+
+sub put_file {
+    my $bytes = 1048576; # 1 MB
+    foreach my $file (@_) {
+        open my $fh, '>', $file
+            or die "Failed to open file $file for writing: $!";
+        for (my $bytes_written = 0; $bytes_written < $bytes; ++$bytes_written) {
+            print $fh 'A';
+        }
+    }
+}
diff --git a/t/freed-bytes.t b/t/freed-bytes.t
new file mode 100644
index 0000000..7d686c0
--- /dev/null
+++ b/t/freed-bytes.t
@@ -0,0 +1,26 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 3;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $tarball_dir = &prep_tar;
+my $test_dir = "$tarball_dir/t/freed-bytes";
+my @files = (
+    "$test_dir/1",
+    "$test_dir/2",
+    "$test_dir/3",
+    "$test_dir/4",
+);
+
+# Smoke test
+ok !&are_hardlinked(@files), 'not hardlinked before we start';
+my (undef, $stdout, $stderr) = &run_script_capture('-f', $test_dir, $test_dir);
+ok &file_exists(@files), 'files were not accidentally deleted';
+ok "freed 24 bytes (24 B)\n" eq $stderr, 'prints correct number of freed bytes';
diff --git a/t/funcs.sh b/t/funcs.sh
deleted file mode 100644
index d25804c..0000000
--- a/t/funcs.sh
+++ /dev/null
@@ -1,99 +0,0 @@
-#!/bin/bash
-# vi: et sts=4 sw=4 ts=4
-
-echo_success() {
-    printf '[\033[1;32m%s\033[0;39m] %s' \
-        '  OK  ' \
-        "$(escape_nonprinting "$*")"
-}
-
-echo_failure() {
-    printf '[\033[1;31m%s\033[0;39m] %s' \
-        'FAILED' \
-        "$(escape_nonprinting "$*")"
-}
-
-echo_warning() {
-    printf '[\033[1;33m%s\033[0;39m] %s' \
-        'WARNING' \
-        "$(escape_nonprinting "$*")"
-}
-
-echo_passed() {
-    printf '[\033[1;33m%s\033[0;39m] %s' \
-        'PASSED' \
-        "$(escape_nonprinting "$*")"
-}
-
-escape_nonprinting() {
-    echo "$*" |cat -v
-}
-
-assert_equals() {
-    local \
-        STARTER=$1 \
-        NEXT
-    shift 1
-    for NEXT; do
-        if [[ $STARTER != "$NEXT" ]]; then
-            echo_failure "$STARTER does not equal $NEXT"
-            echo
-            return 1
-        fi
-        echo_success "$STARTER equals $NEXT"
-        echo
-    done
-}
-
-assert_file_exists() {
-    local FN
-    for FN; do
-        if [[ ! -e $FN ]]; then
-            echo_failure "$FN does not exist"
-            echo
-            return 1
-        fi
-    done
-}
-
-assert_hardlinked() {
-    local \
-        STARTER=$1 \
-        NEXT
-    shift 1
-    for NEXT; do
-        if [[ ! $STARTER -ef $NEXT ]]; then
-            echo_failure "$STARTER is not hard-linked to $NEXT"
-            echo
-            return 1
-        fi
-        echo_success "$STARTER is hard-linked to $NEXT"
-        echo
-    done
-}
-
-assert_nothardlinked() {
-    local \
-        STARTER=$1 \
-        NEXT
-    shift 1
-    for NEXT; do
-        if [[ $STARTER -ef $NEXT ]]; then
-            echo_failure "$STARTER is hard-linked to $NEXT"
-            echo
-            return 1
-        fi
-        echo_success "$STARTER is not hard-linked to $NEXT"
-        echo
-    done
-}
-
-assert_older_than() {
-    if [[ $1 -ot $2 ]]; then
-        echo_failure "$1 is older than $2"
-        echo
-        return 1
-    fi
-    echo_success "$1 is not older than $2"
-    echo
-}
diff --git a/t/link-counting.t b/t/link-counting.t
new file mode 100644
index 0000000..e8d167c
--- /dev/null
+++ b/t/link-counting.t
@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 3;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $tarball_dir = &prep_tar;
+my $test_dir = "$tarball_dir/t/link-counting";
+my @files = (
+    "$test_dir/most-links",
+    "$test_dir/second-most-links",
+);
+
+# Smoke test
+ok !&are_hardlinked(@files), 'not hardlinked before we start';
+&run_script($test_dir);
+ok &file_exists(@files), 'files were not accidentally deleted';
+ok &are_hardlinked(@files), 'files with existing links got hardlinked';
diff --git a/t/normal-linkage.t b/t/normal-linkage.t
new file mode 100644
index 0000000..328f1fa
--- /dev/null
+++ b/t/normal-linkage.t
@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 3;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $tarball_dir = &prep_tar;
+my $test_dir = "$tarball_dir/t/normal";
+my @files = (
+    "$test_dir/foo/same",
+    "$test_dir/same",
+);
+
+# Smoke test
+ok !&are_hardlinked(@files), 'not hardlinked before we start';
+&run_script($test_dir);
+ok &file_exists(@files), 'files were not accidentally deleted';
+ok &are_hardlinked(@files), 'files with the same contents got hardlinked';
diff --git a/t/normal-non-linkage.t b/t/normal-non-linkage.t
new file mode 100644
index 0000000..40377fd
--- /dev/null
+++ b/t/normal-non-linkage.t
@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 3;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $tarball_dir = &prep_tar;
+my $test_dir = "$tarball_dir/t/normal";
+my @files = (
+    "$test_dir/foo/same",
+    "$test_dir/not-same",
+);
+
+# Smoke test
+ok !&are_hardlinked(@files), 'not hardlinked before we start';
+&run_script($test_dir);
+ok &file_exists(@files), 'files were not accidentally deleted';
+ok !&are_hardlinked(@files), 'files with different contents did not get hardlinked';
diff --git a/t/run.sh b/t/run.sh
deleted file mode 100755
index f8dd97e..0000000
--- a/t/run.sh
+++ /dev/null
@@ -1,125 +0,0 @@
-#!/bin/bash
-# vi: et sts=4 sw=4 ts=4
-
-WORKDIR=${0%/*}
-. "$WORKDIR/funcs.sh"
-SCRIPT=${SCRIPT:-$(realpath -- "$WORKDIR/../simplify_static_dir-main.pl")}
-TAR=$(realpath -- "$WORKDIR/t.tar")
-TEMPDIR=$(mktemp -d -t "${0##*/}.XXXXXX")
-
-_prep_tar() {
-    rm -rf t &&
-    tar xf "$TAR"
-}
-cleanup() {
-    rm -rf -- "$TEMPDIR"
-}
-trap 'cleanup' EXIT
-
-test_normal_linkage() {
-    _prep_tar &&
-    $SCRIPT t || return 2
-    local -r \
-        FILE1=t/normal/foo/same \
-        FILE2=t/normal/same
-    assert_file_exists "$FILE1" "$FILE2" &&
-    assert_hardlinked "$FILE1" "$FILE2"
-}
-
-test_normal_nonlinkage() {
-    _prep_tar &&
-    $SCRIPT t || return 2
-    local -r \
-        FILE1=t/normal/foo/same \
-        FILE2=t/normal/not-same
-    assert_file_exists "$FILE1" "$FILE2" &&
-    assert_nothardlinked "$FILE1" "$FILE2"
-}
-
-test_sha1collision_nonlinkage() {
-    _prep_tar &&
-    $SCRIPT t || return 2
-    local -r \
-        FILE1=t/sha1-collision/shattered-1.pdf \
-        FILE2=t/sha1-collision/shattered-2.pdf
-    assert_file_exists "$FILE1" "$FILE2" &&
-    assert_nothardlinked "$FILE1" "$FILE2"
-}
-
-test_zero_size_nonlinkage() {
-    _prep_tar &&
-    $SCRIPT t || return 2
-    local -r \
-        FILE1=t/zero-size/empty1 \
-        FILE2=t/zero-size/empty2
-    assert_file_exists "$FILE1" "$FILE2" &&
-    assert_nothardlinked "$FILE1" "$FILE2"
-}
-
-test_link_counting() {
-    _prep_tar &&
-    $SCRIPT t || return 2
-    local -r FILES=(
-        t/link-counting/{most-links,second-most-links}
-    )
-    assert_file_exists "${FILES[@]}" &&
-    assert_hardlinked "${FILES[0]}" "${FILES[1]}"
-}
-
-test_timestamp_preservation() {
-    _prep_tar &&
-    $SCRIPT t || return 2
-    local -r \
-        FILE1=t/timestamp-preservation/newer-more-linked \
-        FILE2=t/timestamp-preservation/older-less-linked
-    assert_file_exists "$FILE1" "$FILE2" &&
-    assert_hardlinked "$FILE1" "$FILE2" &&
-    assert_older_than "$FILE2" "$FILE1"
-}
-
-test_freed_bytes() {
-    _prep_tar &&
-    local -r OUT=$($SCRIPT -f t/freed-bytes{,,} 2>&1 |tail -1)
-    local -r FILES=(
-        t/freed-bytes/{1,2,3,4}
-    )
-    assert_file_exists "${FILES[@]}" &&
-    assert_equals "$OUT" "freed 24 bytes (24 B)"
-}
-
-test_freed_bytes_commas() {
-    _prep_tar &&
-    local -r OUT=$($SCRIPT -f t/output-commas 2>&1 |tail -1)
-    assert_equals "$OUT" "freed 1,048,576 bytes (1 MB)"
-}
-
-cd "$TEMPDIR" || exit
-TEST_COUNT=0
-TESTS_PASSED=0
-for TESTNAME in \
-    test_normal_linkage \
-    test_normal_nonlinkage \
-    test_sha1collision_nonlinkage \
-    test_zero_size_nonlinkage \
-    test_link_counting \
-    test_timestamp_preservation \
-    test_freed_bytes \
-    test_freed_bytes_commas \
-    ; do
-
-    (( TEST_COUNT++ ))
-    if $TESTNAME; then
-        (( TESTS_PASSED++ ))
-    else
-        echo_failure "$TESTNAME failed"
-        echo
-    fi
-
-done
-
-cleanup
-
-printf '%d/%d tests passed\n' "$TESTS_PASSED" "$TEST_COUNT"
-if [[ $TESTS_PASSED -ne $TEST_COUNT ]]; then
-    exit 1
-fi
diff --git a/t/sha1collision-non-linkage.t b/t/sha1collision-non-linkage.t
new file mode 100644
index 0000000..7fc53ec
--- /dev/null
+++ b/t/sha1collision-non-linkage.t
@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 3;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $tarball_dir = &prep_tar;
+my $test_dir = "$tarball_dir/t/sha1-collision";
+my @files = (
+    "$test_dir/shattered-1.pdf",
+    "$test_dir/shattered-2.pdf",
+);
+
+# Smoke test
+ok !&are_hardlinked(@files), 'not hardlinked before we start';
+&run_script($test_dir);
+ok &file_exists(@files), 'files were not accidentally deleted';
+ok !&are_hardlinked(@files), 'files with the same SHA-1 hash did not get hardlinked';
diff --git a/t/t.tar b/t/t.tar
index a02062a..4273b4a 100644
Binary files a/t/t.tar and b/t/t.tar differ
diff --git a/t/timestamp-preservation.t b/t/timestamp-preservation.t
new file mode 100644
index 0000000..ca5929e
--- /dev/null
+++ b/t/timestamp-preservation.t
@@ -0,0 +1,26 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 4;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $tarball_dir = &prep_tar;
+my $test_dir = "$tarball_dir/t/timestamp-preservation";
+my @files = (
+    "$test_dir/newer-more-linked",
+    "$test_dir/older-less-linked",
+);
+
+# Smoke test
+ok !&are_hardlinked(@files), 'not hardlinked before we start';
+my $should_have_mtime = &filemtime($files[1]);
+&run_script($test_dir);
+ok &file_exists(@files), 'files were not accidentally deleted';
+ok &are_hardlinked(@files);
+ok &has_mtime($should_have_mtime, @files), 'timestamps updated to use oldest';
diff --git a/t/zero-size-non-linkage.t b/t/zero-size-non-linkage.t
new file mode 100644
index 0000000..9093d35
--- /dev/null
+++ b/t/zero-size-non-linkage.t
@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+# vi: et sts=4 sw=4 ts=4
+use strict;
+use warnings;
+
+use Test::Simple
+    tests => 3;
+
+use FindBin qw//;
+use lib $FindBin::RealBin;
+use TestFunctions;
+
+my $tarball_dir = &prep_tar;
+my $test_dir = "$tarball_dir/t/zero-size";
+my @files = (
+    "$test_dir/empty1",
+    "$test_dir/empty2",
+);
+
+# Smoke test
+ok !&are_hardlinked(@files), 'not hardlinked before we start';
+&run_script($test_dir);
+ok &file_exists(@files), 'files were not accidentally deleted';
+ok !&are_hardlinked(@files), 'zero-sized files did not get hardlinked';