If it's the use of hashes that is new to you then Perl Intro and References are good places to start.

Use of the default loop variable $_ in the last code block is explained in Perl Variables

For example, in your code

foreach my $line ( @B ) {
   $k++;
   my $idx1= @$line[0].@$line[1].@$line[2];
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could be written using the default variable $_ as

foreach ( @B ) {
   $k++;
   my $idx1 = $_->[0].$_->[1].$_->[2];
[download]

poj