G'day learnedbyerror,

You can conditionally load modules using the if pragma.

Command line options will be processed at runtime, which will be too late to affect compile-time loading of modules. However, you can modify the command line in a different way, by setting an environment variable, which will be available at compile time.

Here's a minimal example (pm_1155832_cond_use_mod.pl) using List::Util:

#!/usr/bin/env perl -l

use strict;
use warnings;

# Compile time
use if $ENV{PM_1155832_USE}, 'List::Util' => qw{max};

BEGIN {
    print 'Check for List::Util::max() at compile time';
    eval { max(1, 2) };
    print $@ if $@;
}

# Runtime
if (! $ENV{PM_1155832_USE}) {
    require List::Util;
    List::Util->import(qw{max});
}

print 'Max is ', max(1, 2);
[download]

Load module at runtime:

$ pm_1155832_cond_use_mod.pl
Check for List::Util::max() at compile time
Undefined subroutine &main::max called at ./pm_1155832_cond_use_mod.pl
+ line 11.

Max is 2
[download]

Load module at compile time:

$ PM_1155832_USE=1 pm_1155832_cond_use_mod.pl
Check for List::Util::max() at compile time
Max is 2
[download]

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