use constant DEBUG => $ENV{DEBUG_LEVEL} // 0;        # Handle undef gracefully

...

sub helpful {
    my $int = shift;
    debug(1, "called with: ", $int) if DEBUG > 0;    # <-- Add this
    ## do stuff here ...
    my $val = $int * $int;
    debug(3, "output: ", $val)      if DEBUG > 0;    # <-- Add this
    print "$int becomes: $val \n";
}

...