This isn't a complete solution, but I think it will get you started on the right path. If you need more detailed help, I'll need a more detailed description of the problem (namely, expected inputs and outputs). What you want to do is iterate over the string and replace the current character with possible replacement characters. You could use recursion as mentioned above, but that's probably overkill for what you're looking for.

use strict;

#the substitution possibilities
my @a = ('a');
my @b = ('d','e');
my @c = ('f','g','h');

my $string = pop(@_);  #this is an argument passed to the script on th
+e command line
#my $string = "abc";   #if you want it hardcoded

my $expanded_string = "";
my @strings = ();
foreach my $first (@a){
  foreach my $second (@b){
    foreach my $third (@c){
      $string = "$first$second$third";
      push(@strings, $string); #to store them all
      print "$string\n"; #to print this particular one
    }
  }
}
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