I was wondering if someone was going to bring this up. To me, this is the best way to share a variable between two (or more) subroutines.

Just bear in mind, though, that you may receive puzzling errors regarding $new_mail being undefined, depending on where this code appears in the file, and how and when control gets passed to update_mail_list() or read_mail_list().

Best practices would suggest that rathing than enclosing the variable and the referring subs in bare { } braces, you should use a BEGIN { } block. This will ensure that $new_mail gets set up on the lexical pad before you get around to using it.

print@_{sort keys %_},$/if%_=split//,'= & *a?b:e\f/h^h!j+n,o@o;r$s-t%t#u'