See perlop.

Actually, see perlre.

If you're going to put a ] or - character into your character class then it has to be either escaped or be placed at the front after the optional ^,

Or placed at the end immediately before the closing square bracket.

The relevant passage from perlre:

     If you want either "-" or "]" itself to be a member of a
     class, put it at the start of the list (possibly after a
     "^"), or escape it with a backslash.  "-" is also taken
     literally when it is at the end of the list, just before the
     closing "]".
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-sauoq
"My two cents aren't worth a dime.";