On unixish shells, it is the shell, not your program that expands globs. The customary approach to handle this situation is to process @ARGV (instead of @files) after GetOpt has taken all switches away:
my @files = @ARGV;
... rest of program ...
The alternative approach would be to quote the glob on the command line to prevent the shell from expanding it:
> myprog.pl --filespec '*.mp3'
... and afterwards expand it in your program using File::Glob or whatever globbing semantics you want to provide.