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:

GetOptions(
...
);
my @files = @ARGV;

... rest of program ...
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The alternative approach would be to quote the glob on the command line to prevent the shell from expanding it:

> myprog.pl --filespec '*.mp3'
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... and afterwards expand it in your program using File::Glob or whatever globbing semantics you want to provide.