This is not an optimization, but for the sake of TIMTOWTDI:

#!/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) ;

my %dispatch = 
(
    "a" => sub { push @a, shift },
    "b" => sub { push @b, shift },
);


foreach my $value (@temp) 
{

    if ( $value =~ /^([ab])_(.+)$/ )
    {
        my $sub = $dispatch{$1};
        &$sub($2);
    }
}

print "@a <==> @b\n";
[download]