#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11159717
use warnings;

$_ = "a b=c";
/(a)(.*?)(?|b=(\w)|()\z)/ or die;;

printf "\$2 is [%s]\n", defined $2 ? $2 : "undefined";
printf "\$3 is [%s]\n", defined $3 ? $3 : "undefined";

$_ = "a c=c";
/(a)(.*?)(?|b=(\w)|()\z)/ or die;;

printf "\$2 is [%s]\n", defined $2 ? $2 : "undefined";
printf "\$3 is [%s]\n", defined $3 ? $3 : "undefined";
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Outputs:

$2 is [ ]
$3 is [c]
$2 is [ c=c]
$3 is []
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