On line 23 you have "my $pl = ''". You probably intend to use that variable to hold the "previous line." But it's never being set to anything within the while loop.

The standard algorithm is something like this:

my $prev;
while( my $curr = <$infile> ) {
  chomp $curr;
  if( defined $prev && $curr eq $prev ) {
    # Take some action.
  }
  $prev = $curr;
}
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If you need to report the line number within the file of the contents of $prev, it's always going to be $.-1.