#!/usr/bin/perl
use strict;
use warnings;
use Getopt::Long;


GetOptions(
    'debug:s' => \my $debug,
) or die;
if (defined ($debug)) {
    $debug = {map {$_, 1} split /,/, $debug};
}

sub debug {
    my ($msg) = @_;
    my $caller = (caller(1))[3];
    warn "$msg\n" if exists $debug->{$caller};
}

sub foo {
    debug('foo called');
}

sub bar {
    debug('bar called');
}

foo();
bar();