Unless I much mistake, you have some quotage problems. Specifically, you need double quotes, like so:

@ARGV = ("$mail_directory/users/$table_ext/$user\@domain.com")
unless @ARGV;
[download]

Double quotes interpolate variables, single quotes don't.

 
perl -e 'print "I love $^X$\"$]!$/"#$&V"+@( NO CARRIER'