use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent = 0;

my $string_one = 'foo bar [a21] plus (b23) baz bax';
my @string_one_results = $string_one =~ m { (a|b) (\d+) }gx;
print "RESULTS: " , Dumper(\@string_one_results), $/;
# RESULTS: $VAR1 = ['a','21','b','23'];