my @temp = (...);
my @a = map {/a_(.*)/ ? $1 : ()} @temp;
my @b = map {/b_(.*)/ ? $1 : ()} @temp;

##</code><code>##

my @a = map {/a_(.*)/; $1} grep /a_/ @temp;
my @b = map {/b_(.*)/; $1} grep /b_/ @temp;