#!/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) ;
foreach my $value (@temp) {
    push @a, $1 if ($value =~ /a_(.*)/) ;
    push @b, $1 if ($value =~ /b_(.*)/) ;
}
print "@a <==> @b";