Another way that works from a string instead of from an array:

use strict;
use warnings;

my @temp = qw( a_1 b_1 a_2 a_3 a_4 b_2 a_5 b_3 a_6 b_4 );

local $_ = join(' ', @temp);
my @a = /a_(\d+)/g;   # or: my @a = /\ba_(\d+)\b/g;
my @b = /b_(\d+)/g;   # or: my @b = /\bb_(\d+)\b/g;

$, = ", ";
$\ = "\n";
print(@a);
print(@b);

__END__
output
======
1, 2, 3, 4, 5, 6
1, 2, 3, 4
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