In short:

] and - are special in character classes.

As rules of thumb avoiding ambiguity°:

If needed ...

• use ] as first character
• use - as last character

update

D:\>perl -dE0

Loading DB routines from perl5db.pl version 1.49_05
Editor support available.

Enter h or 'h h' for help, or 'perldoc perldebug' for more help.

  DB<1> say "$_:\t", $_ =~ /[]abc-]/ for qw/] [ - a/
]:      1
[:
-:      1
a:      1

  DB<2>
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°) Otherwise ] will close the class and - will denote a range.