According to Camel (3rd) on p. 85, it says:

    @files = glob $some_pattern;    # Call glob as an operator.
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When I first read that, I thought $some_pattern meant $some_regex. But, it doesn't. (Or, rather, it does mean $some_regex, but it means to use a regex with a different syntax than Perl's regex syntax.)

What the OP is looking for probably falls under the category of:

my @files =
    grep { /[A-Za-z]{2}_\w{2,5}/ }
        glob "??_??";
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Two different regex syntaxes. *shrugs*