#!/usr/bin/perl -w
use strict;
my @temp=("a_1","b_1","a_2","a_3","a_4","b_2","a_5","b_3","a_6","b_4");

my (@a, @b) ;
m[^([ab])_(.*)$] and push @{$1 eq 'a' ? \@a : \@b}, $2 for @temp;
print "@a <==> @b";