Apache resp. mod_perl is wrapping your CGI script in a subroutine to repeatedly call it.

sub handler {
   #!/usr/bin/perl
   use strict;
   use warnings;
   use CGI;
   my $cgi = new CGI;
   print_document_header();
   sub print_document_header {
      print $cgi->header( -type => 'text/html' ), "\n";
   }
}
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Your workaround of explicitly passing around the variable is a good one in my opinion.

you can just use our $cgi instead of my $cgi

Locutus: Please, avoid globals - the "best" way to do it is to pass $cgi as an argument to the subroutines.

I'd say the problem is inter-mixing lexicals my $cgi with package symbols like named subs like sub print_document

Changing to lexicals holding anonymous sub refs my $print_document = sub {} would solve the OP'S problem too.

Having persistent shared globals can be a desired effect. (Initialization overhead, communication channel)

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!}

This is getting a bit off-topic, but some globals are ok. Novice programmers don't always know when they're safe and when they aren't, and they're often too timid to change a global to a local when it really needs to be done. That's why we advise people not to use globals at all. But CGI is probably safe as a global, because there's no reason to have two of them in your program.