Getopt works on @ARGV, so you can't use it to process function arguments. Well, not without doing @ARGV = @_ which might well break other stuff.

You can try something like this:

#!/usr/bin/perl

use strict;
use warnings;
use Getopt::Std;
use Data::Dumper;


sub getopt_fun {
    my %args;

    {
        local @ARGV = @_;
        getopt 't', \%args;
    }

    print Dumper \%args;
}

getopt_fun (-t => 'foo');
[download]

Edit: narrowed the scope of the local.