#!/usr/local/bin/perl

use strict;
use warnings;

my @parents = (0);

my $prev_tabs = 0;

while (<DATA>) {
  chomp;
  my $tabs = tr/\t//d;

  if ($tabs > $prev_tabs) {
    push @parents, $. - 1;
  } elsif ($tabs < $prev_tabs) {
    $#parents = $tabs;
  }

  print "$. : $_ : $parents[-1]\n";

  $prev_tabs = $tabs;
}

__END__
cat1
        cat1,sub1
                cat1,sub1,sub-sub1
        cat1,sub2
cat2
cat3
        cat3,sub1