#!/usr/bin/perl -l

# https://en.wikipedia.org/wiki/Permutation#Generation_in_lexicographic_order

use strict;
use warnings;

$_ = 'aabbcc';

1 while print,
  s/.*\K # find the last
  (.) # char such that
  (.*) # there is a later (latest)
  (.)(??{$1 ge $3 and '(*F)'}) # char greater than it
  (.*) # and get rest
  # swap those two chars ( $1 & $3 )
  # then reverse everything after the first swapped char
  / $3 . reverse $2.$1.$4 /xe