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

my %parts;
push @{ $parts{ substr $_, 0, 1 }}, substr $_, 2
  foreach @temp;

use Data::Dumper;
print Dumper \%parts;