my %hash;

# Assume a loop here that creates keys and values... 

my $key = 'alphabet';
my $letters = ['abc','def'];  

if (exists $hash{$key}) {
     my @letters_array = $hash{$key};
     push @letters_array, $letters;
     my $new_val = [@letters_array];
     $hash{$key} = $new_val;
 }
 else {
   $hash{$key} = $letters;
 }
# Then check for "duplicate" keys (ie. a key with more than one value).

for $key (keys %found_mod) {
  if (ref($found_mod{$key}[0]) eq 'ARRAY') {
    print "This key -- $key -- has more than one value!\n";
  }
}