Hello smile4me,

1. does the perl compiler optimize away the debug's when the ENV{DEBUG_LEVEL} is 0?

No.

2. is there a cost to leaving the debug()'s inline?
3. should I remove them to improve performance overall?

Yes, there is a cost, but it’s probably negligible. If performance becomes a problem, you should first profile the code using a module such as Devel::NYTProf to find the bottlenecks. Unless your debug statements are placed inside heavy-duty loops, it’s unlikely they will degrade performance significantly.

4. is there a better way to accomplish the goal of simple on/off for debugging one's code?

If you want the compiler to eliminate debugging code when debugging is not required, you need to code like this:

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

...

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";
}

...
[download]

You can check with perl -MO=Deparse that when DEBUG == 0, the debug statements are removed by the compiler.

Hope that helps,