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

##</code><code>##

$b =~ s/b/randme()/g;

##</code><code>##

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()