open my $idx, '<', 'index.file' or die "Can't open 'index.file' for reading: $!";
my %index;
while (<$idx>){
    chomp;
    $index{$_} = 1;
}

##</code><code>##

while (<$in>) {
    if ($index{$.}){
        s/foo/bar/g;
    }
    print $tmp $_;
}