Hello First, please, read about file handling in perl and
think after what $file='STDIN' means.
check this version of your method

#! /usr/bin/perl

use strict;
use warnings;

use strict;
use FileHandle;

sub slurp {
    my $fh = new FileHandle;
    my $file = shift;
    local undef($/);
    if (defined($file)) {
      $fh->open($file) or die "Can't open $file: $!";
    } else {
      $fh->fdopen(*STDIN, 'r');
    }
    my $slurp = <$fh>;
    $fh->close or die "Can't close $file: $!";
    $slurp;
}

print slurp();
[download]

Alex