You can use shell redirection if you like.

./myscript.pl > demo.txt
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Or you could modify the script itself:

#!/usr/bin/perl

use strict;
use warnings;

open my $fh, '>', 'demo.txt' or die $!;

print $fh "Content-type: text/html\n\n";

foreach my $key (sort keys %ENV) {
    print $fh "\$ENV{$key} = $ENV{$key}<br />\n";
}

close $fh or die $!;
[download]

I suggest you spend 30 minutes reading perlintro. If this is homework, you're going to need to actually learn this, and perlintro is a good start.