Personally I prefer the (TEST)?TRUE:FALSE method (C many many years ago) but extending the idea of using a list (but not clarity):-

($a,$throw_away) = (("abc" =~ /a(b)c/) , 'd');
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if the RegEx doesn't match then you have a single element list containing 'd', if it does then 1st element is still 'b' the $throw_away variable just captures any remaining elements.

or

$a = ( ("abc" =~ /a(b)c/) , 'd' )[0];
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Taking this idea to an extreme, what about pulling set of values from the regex or setting all the variables to a set value.

($a,$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
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