Or we can use the addfile method for more efficient reading of the file:

use Digest::MD5;

    my $file = shift || "/etc/passwd";
    open(FILE, $file) or die "Can't open '$file': $!";
    binmode(FILE);

    print Digest::MD5->new->addfile(*FILE)->hexdigest, " $file\n";

#!/usr/bin/perl

use warnings;
use strict;
use Digest::MD5;

my $su = 'be90385edbb12bf9982932902f10c4d5';


open(FILE, "/bin/su");                                  # nervig .oO
my $md5 = Digest::MD5->new;
while (<FILE>) {
  $md5->add($_);
}

close(FILE);


if($md5->hexdigest ne $su) {
  print "Warning, files were modified :/\n";
} else {
  print "No modification :)\n";
}

#!/usr/bin/perl

use warnings;
use strict;
use File::stat;

my $file = '/bin/su';
my $oldtime = 'Sun Apr 23 08:48:54 2006';

my $sb = stat($file) || die "something went wrong :/";

my $newtime = scalar localtime $sb->mtime;

if($newtime ne $oldtime) {
  print "Warning, file was modified!\n";
} else {
  print "no modification\n";
}