To make it global beyond "within the current package":

sub ::DEBUG() { 1; }

# Example use:
DebugDump( \%hash ) if ::DEBUG();
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As for how to do this in Apache, I'd probably just make "use DEBUG;" do this for you and then you can tell Apache to load the DEBUG module when you want this.

Update: But you'll want sub ::DEBUG() { 0; } when you don't want debugging so you probably need two different ways to "use DEBUG". You could have "use DEBUG;" turn on debugging (for easy use in an Apache config). Then "use DEBUG qw/inherit off/;" would only define ::DEBUG() if it hadn't already been defined (in which case it would define it as 0 or 1, depending on whether 'off' or 'on' was the second argument). So any code that uses ::DEBUG() would have the "use DEBUG qw/inherit off/;" near the top so that ::DEBUG() wouldn't try to call a non-existant subroutine.

package DEBUG;
use strict;
sub import
{
    my $pkg= shift @_;
    my $default= 1;
    if(  @_  &&  'inherit' eq $_[0]  ) {
        return
            if  defined \&::DEBUG;
        shift @_;
        $default= 0;
    }
    if(  @_  &&  $_[0] =~ /^(on|off)$/  ) {
        $default= 'on' eq $_[0] ? 1 : 0;
        shift @_;
    }
    if(  @_  ) {
        require Carp;
        Carp::croak( "Usage: use DEBUG ['inherit',] ['on'|'off'];" );
    }
    eval "sub ::DEBUG() { $default; }; 1"
        or  die $@;
}
sub unimport
{
    if(  2 < @_  ||  2 == @_ && 'inherit' ne $_[1]  ) {
        require Carp;
        Carp::croak( "Usage: no DEBUG ['inherit'];" );
    }
    import(@_,'off');
}
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