Under mod_perl when working with external modules and most templating/frameworks I have found code similar to this helpful:

# OO method for error to STDERR uisng 'warn'
# which under mod_perl/Apache is your
# default error_log

sub error_to_log {
    my ($self,$error,$override) = @_;
        if ($self->_debug() || $override) {
        warn "$error\n";
    }
}
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So if your _debug method is toggled on this comment would get logged:

$object->error_to_log("I made it this far, I can't believe it");
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If you don't "advertise"* the _debug method or you are doing a minor debug you could simply call it with:

$object->error_to_log("Date: $date Variable: $var",1);
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The '1' simply forces the error message to be written.

There are many more robust solutions available, but I have found this method to be very useful and allows me control over the error logging process. Since it is abstracted(?) enough I can make changes to the logging method without recoding other sections of the application.

So you don't want OO?

# $debug is a global variable
# you will need to assign

sub error_to_log {
    my ($error,$override) = @_;
        if ($debug || $override) {
        warn "$error\n";
    }
}
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Then you call it like any other sub:

&error_to_log("Error message");
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* In Perl it is not as common to make a method completely private, as it may be in languages like Java. In most cases they are still acessible if you know they exist, that is why I prefer the "advertise" verbiage. Ways to make a method private can be found in other nodes, but I didn't bother to look them up because this node is about logging not private vs. public methods.