file name as argument and sort in descending way in perl?

I have this perl script

#!/usr/bin/perl
use strict; 
open my $df, '<', 'file.txt' or die "open: $!\n";
my %h;
while (<$df>) {
    $h{$_}++ for split /\s+/;
    END{
        for (keys %h){
            print $_, "=", $h{$_};
            print "\n";
        }
     } 
}

it prints the occurrence of each word in the file, I have several questions

1) How can I make this script dynamic by let the user put one argument as a file name

Now:

perl script.pl

wanted:

perl script.pl fileName

2) How can I sort the output in descending way?