I think you can do this with the .. operator but I'm not sure if this is the best way. Maybe this will help, though:

#!/usr/bin/perl -w
use strict ;

if (open (my $file, '/testfile')) {
  while (<$file>) {
    print if 1../^## START/ ;
  }
  close $file ;
}
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Update: Oops, forgot to mention, the .. operator is documented in perlop. Thanks c.