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 the 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
    }
  }
}