Getopt::Long::GetOptions
(
    'f=s{1,}' => sub
    {
        print "This is the number of \@ARG arguments inside the process " . @ARGV . "\n";
        print "This is the number of \@xls arguments inside the process " . @xls  . "\n";
        push @xls, pop @_;    # <-- Add this
    },
    ...