You can only override some builtins, and those you can override must be overridden by importing the replacement functions from another module.

>type TestHarness.pm

use strict;
use warnings;

package TestHarness;

BEGIN {
   our @EXPORT_OK = qw( unlink );
   require Exporter;
   *import = \&Exporter::import;
}

sub unlink {
   print("PRE unlink\n");
   # Don't forget to properly handle context.
   my $rv = unlink(@_);
   print("POST unlink\n");
   return $rv;
}

1;
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>type test.pl

use strict;
use warnings;

BEGIN {
   # Parse options or something.
   $::TESTING = 1;
}

use if $::TESTING, TestHarness => qw( unlink );

unlink();
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>perl test.pl

PRE unlink
POST unlink
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You could control TestHarness::unlink's behaviour using globals.