sub Diff {
    my @a;
    my $dif = undef();
    for my $fdx (0..1) {
        open my $fh, $_[$fdx] or die "$!: $_[$fdx]\n";
        my @tmp = <$fh>;
        $a[0] = \@tmp;
        close $fh;
    }
    while ( $a[0] -> [0] and $a[1] -> [0] ) {
        if ( $a[0] -> [0] eq $a[1] -> [0] ) {
            shift @$a[0]; shift @$a[1];
        }
        elsif ( $a[0] -> [0] ) {
            $dif .= '< ' . shift( @$a[0] );  
        }
        else {
            $dif .= '> ' . shift( @$a[1] );
        }
    }
    return $dif;
}