#!/usr/bin/perl
use strict; use warnings; use 5.010;

my @az = 'a'..'z';
my %cache = ( a => 1, b => 1, c => 1 );

for (1..20) {
  my $key = each %cache || ( say "back to front!" and each %cache );
  say $key;
  delete $cache{$key};
  $key = $az[rand @az] . $key;
  $cache{$key} = 1;
}
##</code><code>##

c
a
b
sa
qsa
zc
back to front!
zqsa
zzc
wb
izzc
ezqsa
cizzc
nwb
nnwb
eezqsa
back to front!
icizzc
bnnwb
neezqsa
back to front!
sneezqsa
jicizzc