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use strict;
use warnings;
...
my @string_three_results = $string_three =~ / (?| (\d+) (a|b) | (a|b) 
+(\d+) ) /gx;

print 'RESULTS: ', Dumper(\@string_three_results), $/;

16:38 >perl 2020_SoPW.pl
RESULTS: $VAR1 = ['21','a','b','23'];

16:38 >