With only 1 mismatch, just throw all the patterns together and let Perl's regex engine have at it: 
$pat = join '|', reverse sort keys %pattern;
$pat = qr/$pat/;

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Any vaguely recent Perl has the "trie optimization" which will automatically merge the branches. You can run with perl -Mre=debug to see what's going on.

EDIT: For more than 1-2 mismatches, you'll need to do something a bit fancier, either constructing a suffix tree or using seeds (you can show that any k-mismatch pattern must contain a substring of your original pattern of at least a certain length). I don't know of a Perl module off-hand to do either.

In reply to Re: generating hash patterns for searching with one mismatch by educated_foo
in thread generating hash patterns for searching with one mismatch by cedance

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