#!/usr/bin/env perl
use constant DEBUG => $ENV{DEBUG_LEVEL};

sub debug {
    my $level = shift;
    return if ($level > DEBUG);
    print STDERR "($level/" . DEBUG . ") @_\n";
}


sub helpful {
    my $int = shift;
    debug(1, "called with: ", $int);
    ## do stuff here ...
    my $val = $int * $int;
    debug(3, "output: ", $val);
    print "$int becomes: $val \n";
}

for ( 1..5 ) {
  helpful( $_ );
}