> I get the following error

It's not an error, it's a warning. You can easily tell it from the W in the diagnostics output: "(W uninitialized)".

The most probable reason is some of the regexes didn't match anything, so their corresponding element in the array is undefined. You can print 0 instead of an undefined value using the defined-or operator //:

    print {$out} "$regex:\t\t", $tally[$i] // 0, "\n";
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or you can prepopulate the array with zeros:

my @tally = (0) x @regexes;
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