Sometimes it can be better to do it externally.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1130122

use strict;
use warnings;

my $want = undef;
my @idxArray;

# sort files together :)

open my $fh, '-|', '/usr/bin/sort -n d.master d.index' or die "$! open
+ing sort";

while(<$fh>)
  {
  if( /^(\d+)$/ )
    {
    $want = $1;
    }
  elsif( $want && /^$want\s+(\S+)/ )
    {
    push @idxArray, $1;
    $want = undef;
    }
  else
    {
    $want = undef;
    }
  }
close $fh or die "$! on close of sort";

use YAML; print Dump \@idxArray; # for debugging

__END__
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I hope you're on an OS with a decent sort :)