Using glob or the <> operator looks like the ticket. Consider:

#!/usr/bin/perl -w
use strict;

print "$_\n" for (<{f}{r,x,y}{e,a,b}{d}>);

__END__
program:  op's post
fred        fred
frad        fxed
frbd        fyed
fxed        frad
fxad        frbd
fxbd        fxad
fyed        fxbd
fyad        fyad
fybd        fybd
[download]

Update: Thought I had x,y and a,b reversed in previous code. Note the order is different than the op's post, but the combinations are the same.

Continuing on.. a straightforward implementation of a sub to generate the glob string is shown below.. took a couple of more steps than I thought it would.

#!/usr/bin/perl -w
use strict;

my $string = 'fred';
my %substitution = (
    2 => 'xy',
    3 => 'ab'
);
my @variants = gen_variants($string, %substitution);

print join("\n",@variants),"\n";

sub gen_variants
{
   my ($string, %substitution) = @_;
   
   my @letters = split(//,$string);
   
   foreach my $pos (keys %substitution)
   {
      my @newltrs = split(//,$substitution{$pos});
      $letters[$pos-1] = join(',',$letters[$pos-1],@newltrs);
   }
   
   @letters = map{"{$_}"}@letters;
   my $globstr = join('',@letters);
   print "glob string = $globstr\n";
   return (glob($globstr));
}

__END__ 

prints:
glob string = {f}{r,x,y}{e,a,b}{d}
fred
frad
frbd
fxed
fxad
fxbd
fyed
fyad
fybd
[download]