If all you're doing is trying to match a '+', another way to do this would be with the index function:

#!/usr/bin/perl -w
use strict;

my $a = '123a+b456';
print "found\n" unless index($a, 'a+b') == -1;
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I'm not entirely sure how efficient that is, though (but I have a suspicion it's more efficient than attempting a full regex match).