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 %values;
foreach (@temp) {
    /^([ab])_(.*)/ && do {
        push @{$values{$1}}, $2;
        next;
    };

    die "'$_' doesn't match the pattern of ^[ab]_.*\n";
}

print "$_ => @{$values{$_}}\n" for qw( a b );