#! perl -slw
use strict;
use Digest::MD5 qw[md5];

my %h;

open my $f1, '<', $ARGV[0] or die $!;
while( <$f1> ) {
        $h{ md5 $_ } = undef;
}
close $f1;

print "Lines found in $ARGV[1] not seen in $ARGV[0]";

open my $f2, '<', $ARGV[1] or die $!;
while( <$f2> ) {
        print "$.:$_" unless exists $h{ md5 $_ };
}
close $f2;

 __END__
E:\>copy bigfile.dat bigfile2.dat
        1 file(s) copied.
E:\>echo "this line was added to bigfile2" >>bigfile2.dat
E:\>echo "and this line was added to bigfile2" >>bigfile2.dat
E:\>echo "and so was this line added to bigfile2" >>bigfile2.dat
E:\>prompt $T$S$P$G

 4:03:28.61 D:\Perl\test>258709 e:\bigfile.dat e:\bigfile2.dat

Lines found in e:\bigfile2.dat not seen in e:\bigfile.dat

2378882:<START"this line was added to bigfile2"
2378883:"and this line was added to bigfile2"
2378884:"and so was this line added to bigfile2"

 4:11:28.82 D:\Perl\test>