#!/usr/bin/perl

use strict;
use warnings;

my @alphabet = ('a'..'z', 'A'..'Z', '_');
my $min_size = 1;
my $max_size = 6;

my @state = ();
do {
    print @alphabet[@state], "\n" if $min_size <= @state;
    if (@state < $max_size) {
        push @state, 0;
    }
    else {
        for (my $i = $#state; $i >= 0; --$i) {
            last if ++$state[$i] < @alphabet;
            pop @state;
        }
    }
}
while (@state);