#!/usr/bin/env perl

use v5.20;
use warnings;
use feature 'signatures';
no warnings 'experimental::signatures';

# ------ Debugging function definition:
our %debug;
our $d_area = "(default)";
sub dsay( @args ) {
    return
        unless $debug{ uc $d_area }
            || $debug{'ALL'};
    my $prefix =
        $d_area ne "(default)"
        ? "DEBUG $d_area: "
        : "DEBUG: ";
    say $prefix, @args;
}

# ------ Application program:
sub common_sub {
    say "call to common sub (d_area is $d_area)";
    dsay "debugging output from common_sub";
}

sub one {
    say "call to one:";
    local $d_area = "one";
    dsay "debugging output from one";
    common_sub;
    dsay "more debugging output from one";
}

sub two {
    say "call to two:";
    local $d_area = "two";
    dsay "debugging output from two";
    common_sub;
    dsay "more debugging output from two";
}

# Switch on debugging for area 'two'
# (I'll use command line switches with Getopt::Long later).
$debug{TWO} = 1;

say "hello from main";
dsay "debugging output from main";
one;
two;

##</code><code>##

hello from main
call to one:
call to common sub (d_area is one)
call to two:
DEBUG two: debugging output from two
call to common sub (d_area is two)
DEBUG two: debugging output from common_sub
DEBUG two: more debugging output from two

##</code><code>##

#
# Dsay.pm
#
package Dsay;
use parent 'Exporter';
our @EXPORT = qw( %debug $d_area dsay );

our %debug;
our $d_area = "(default)";

sub dsay( @args ) {
    return
        unless $debug{ uc $d_area }
            || $debug{'ALL'};
    my $prefix =
        $d_area ne "(default)"
        ? "DEBUG $d_area: "
        : "DEBUG: ";
    say $prefix, @args;
}

1;

##</code><code>##

use Dsay;

##</code><code>##

    local $Dsay::d_area = '...';