Instead of glob you can also use explicit enumeration of all possibilities. There are 2**n of them if n is the length of your word. If you translate all numbers from 0 to 2**n-1 into binary and translate 0 into the empty string, you get all combinations. The following script compares run time against glob.

use strict;
use warnings;
use Time::HiRes qw[ time ];

# explicit enumeration
my $s = time;
my $string = "aabc";
my $n = length $string;
my $p = 2**$n;
for( my $i=0; $i<$p; $i++ ) {
  my $x = sprintf( "%0${n}b", $i );
  print substr( $string, $_, 1 ), substr( $x, $_, 1) ? "1" : "" for 0.
+.$n-1;
  print "\n";
}
print STDERR "time: ", time-$s, "\n";

# using glob
$s = time;
my $g = "";
$g .= substr( $string, $_, 1 ).'{,1}' for 0..$n-1;
print "$_\n" while( glob $g );
print STDERR "time: ", time-$s, "\n";
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On my machine it seems that explicit enumeration is slightly faster but not much.