I don't recommend this, but if you purposely use a global variable why bother passing it to a subroutine? Global is global.

use strict;
use warnings;
my $global = 1;
mt();
print "C: global still = $global\n";

sub mt {
  print "A: global=$global\n";
  $global++;
  print "B: global=$global\n";
}
__END__

A: global=1
B: global=2
C: global still = 2
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