($a,$throw_away) = (("abc" =~ /a(b)c/) , 'd');

##</code><code>##

$a = ( ("abc" =~ /a(b)c/) , 'd' )[0];

##</code><code>##

($a,$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