For some reason, I thought I had to use a seperate module for the sub () {} optimizations to occur, but the following works fine.

use strict;
use warnings;

use vars qw( $LOG_FILE_NAME );

BEGIN {
   if (@ARGV && $ARGV[0] =~ /^-?-log(?:=(.*))?$/) {
      *LOGGING = sub () { 1 };
      $LOG_FILE_NAME = $1;
   } else {
      *LOGGING = sub () { 0 };
   }
}

if (LOGGING) {
   if (defined($LOG_FILE_NAME)) {
      open(LOG_FH, '>>', $LOG_FILE_NAME)
      or die("Can't open log file: $!\n");
   } else {
      open(LOG_FH, '>&STDERR')
      or die("Can't dup STDERR: $!\n");
   }
}

print LOG_FH ('Opening log file at '.localtime().".$/") if LOGGING;
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perl -MO=Terse script.pl

LISTOP (0x1df5678) leave [1]
    OP (0x1df55fc) enter
    COP (0x1df56c0) nextstate
    OP (0x1ba3544) null [5]
    COP (0x1df563c) nextstate
    OP (0x1df53f8) null [5]
script.pl syntax OK
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