use strict;
use warnings;

use do_compile_time_arg_check;
# Checks if the first arg is --log or --log="file name".
# Imports LOGGING as a constant sub.
# Imports strict-safe $LOG_FILE_NAME.

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;

##</code><code>##

# The first time [[ use do_compile_time_arg_check; ]]
# or [[ use do_compile_time_arg_check (); ]] is used,
# the first command line arg is removed from @ARGV if
# it's [[ --log ]] or [[ --log="file name" ]].

# Whenever [[ use do_compile_time_arg_check; ]] is used
# (including the first time),
#
# - [[ LOGGING ]] is exported as a constant sub. It returns
#   true if [[ --log ]] or [[ --log="file name" ]] was found
#   at the head of the argument list the first time this
#   module was used.
#
# - [[ $LOG_FILE_NAME ]] is exported as a strict-safe global.
#   It contains the file name from [[ --log="file name" ]] if
#   that argument was found at the head of the argument list
#   the first time this module was used.

use strict;
use warnings;

package do_compile_time_arg_check;

use vars qw(
   $log
   $log_file
);

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

sub import {
   my $caller_pkg = caller();

   my $lexical_log = $log;

   {
      no strict 'refs';
      *{"${caller_pkg}::LOGGING"      } = sub () { $lexical_log };
      *{"${caller_pkg}::LOG_FILE_NAME"} = \$log_file;
   }
}

1;

##</code><code>##

>perl -MO=Terse script.pl
Useless use of a constant in void context at script.pl line 10.
Useless use of a constant in void context at script.pl line 20.
LISTOP (0x1df2e3c) leave [1]
    OP (0x1dfb290) enter
    COP (0x1df2e88) nextstate
    OP (0x1ba34e4) null [5]
    COP (0x1dfb2d4) nextstate
    OP (0x1dfb174) null [5]
script.pl syntax OK

##</code><code>##

>perl -MO=Terse script.pl --log
LISTOP (0x1df2644) leave [1]
    OP (0x1df23b0) enter
    COP (0x1df266c) nextstate
    LISTOP (0x1df26c8) leave
        OP (0x1df26a8) enter
        COP (0x1df26f0) nextstate
        UNOP (0x1df272c) null
            LOGOP (0x1df2750) cond_expr
                UNOP (0x1df3e78) defined
                    UNOP (0x1dfb770) null [15]
                        PADOP (0x1ba2fd4) gvsv  1
                LISTOP (0x1dfb1e4) leave
                    OP (0x1df2778) enter
                    COP (0x1dfb20c) nextstate
                    UNOP (0x1dfb248) null
                        LOGOP (0x1dfb26c) or
                            LISTOP (0x1dfb6f0) open [4]
                                OP (0x1dfb6d0) null [3]
                                PADOP (0x1dfb534) gv  3
                                SVOP (0x1dfb718) const  SPECIAL #0 Nullsv
                                UNOP (0x1dfb670) null [15]
                                    PADOP (0x1dfb694) gvsv  2
                            LISTOP (0x1dfb424) die [9]
                                OP (0x1dfb294) pushmark
                                UNOP (0x1dfb478) null [67]
                                    OP (0x1dfb458) null [3]
                                    BINOP (0x1dfb4a0) concat [7]
                                        BINOP (0x1dfb2b4) concat [6]
                                            SVOP (0x1dfb400) const  SPECIAL #0 Nullsv
                                            UNOP (0x1dfb300) null [15]
                                                PADOP (0x1dfb3dc) gvsv  5
                                        SVOP (0x1dfb2dc) const  SPECIAL #0 Nullsv
                LISTOP (0x1df27b8) leave
                    OP (0x1df2798) enter
                    COP (0x1df27e0) nextstate
                    UNOP (0x1df281c) null
                        LOGOP (0x1df2840) or
                            LISTOP (0x1df2a6c) open [11]
                                OP (0x1dfb164) null [3]
                                PADOP (0x1df2a48) gv  10
                                SVOP (0x1dfb188) const  SPECIAL #0 Nullsv
                            LISTOP (0x1df2970) die [16]
                                OP (0x1df2868) pushmark
                                UNOP (0x1df29c0) null [67]
                                    OP (0x1df29a0) null [3]
                                    BINOP (0x1df29e8) concat [14]
                                        BINOP (0x1df2888) concat [13]
                                            SVOP (0x1dfb1c0) const  SPECIAL #0 Nullsv
                                            UNOP (0x1df2928) null [15]
                                                PADOP (0x1df294c) gvsv  12
                                        SVOP (0x1dfb4fc) const  SPECIAL #0 Nullsv
    COP (0x1df23d0) nextstate
    LISTOP (0x1df2570) print
        OP (0x1df2550) pushmark
        UNOP (0x1df240c) rv2gv
            PADOP (0x1df2620) gv  21
        SVOP (0x1df25dc) const  SPECIAL #0 Nullsv
        UNOP (0x1df2598) scalar
            OP (0x1df25bc) localtime [17]
        UNOP (0x1df24b8) null [67]
            OP (0x1df2498) null [3]
            BINOP (0x1df24e0) concat [19]
                SVOP (0x1df2508) const  SPECIAL #0 Nullsv
                UNOP (0x1df2430) null [15]
                    PADOP (0x1df2454) gvsv  18
script.pl syntax OK