Hello ck346, and welcome to the Monastery!

First,

• See How do I compose an effective node title?
• Please put code inside <code> ... </code> tags; see Markup in the Monastery.
• Always use strict; as well as warnings.
• Please review How do I post a question effectively? In particular, instead of saying I have spent all day and can't figure it out, show what you’ve tried and how it failed.

Now to the question of printing to a file. Do this in three stages:

1. Open the file for writing. Use the 3-argument form of open together with a lexical (my) filehandle, and check for errors:

   my $output_file = 'demo.txt';
   
   open(my $out_fh, '>', $output_file)
       or die "Cannot open file '$output_file' for writing: $!";
   [download]

2. Print to the filehandle as you would to the console: just put the filehandle as the first argument to open, with no comma between the filehandle and the next argument:

   print $out_fh "\$ENV{$key} = $ENV{$key}\n";
   [download]

3. Close the filehandle when you’re finished writing to the output file:

   close $out_fh
       or die "Cannot close file '$output_file': $!";
   [download]

See perlopentut.

Hope that helps,