Here's a solution.
I'll leave it to you to determine if it's faster, smaller, better, etc.
#!/usr/bin/perl -l
# http://perlmonks.org/?node_id=1188292
use strict;
use warnings;
my $n = shift // 3; # alphabet size
my $t = shift // $n; # size of tuples
my @alphabet = ('A'..'Z', '0'..'9', 'a'..'z');
my %next;
@next{'', @alphabet} = @alphabet;
$_ = $alphabet[0] x $t; # start of string
my $over = $alphabet[$n]; # outside of alphabet
my $wantedlength = $n ** $t + $t - 1;
while( length $_ >= $t )
{
if( /^(?=(.{$t})).+\1/ ) { s/^./$next{$&}/; }
elsif( s/$over(.)/$next{$1}/ ) { }
elsif( $wantedlength == length $_ )
{
print;
# prove it is correct
my %all;
$all{$1}++ while /(?=(.{$t}))/g;
my $count = keys %all;
print "want @{[ $n ** $t ]} unique strings, have $count";
die "duplicate string" if grep $_ > 1, values %all;
exit;
}
else { $_ = $alphabet[0] . $_; }
}
The loop seems awkward, there ought to a cleaner way...