($a,$b,$throw_away) = (("abcde" =~ /a(b)c(d)/),('UNDEFINED') x 3); 
print $a , "::" , $b ; # prints b::d

and 

($a,$b,$throw_away) = (("abcde" =~ /a(x)c(x)/),('UNDEFINED') x 3); 
print $a , "::" , $b ; # prints UNDEFINED::UNDEFINED