Have you looked closely at the modifiers after each substitution? For example, add an "e" to option 1.

$a =~ s/a/$a[int(rand(@a))]/ge;
[download]

Or, perhaps better yet, take it away from option 2.

$b =~ s/b/randme()/g;
[download]

Result:

option 1:
heebdjfjiecbieiebdfddi

option 2:
randme()randme()randme()randme()randme()randme()randme()randme()
randme()randme()randme()randme()randme()randme()randme()randme()
randme()randme()randme()randme()randme()
[download]

See perlop for the meaning of the flags and some examples.