Two ways.

1. You can tell Perl to use the C preprocessor with the command line option -P. Be sure to read the limitations of the switch, as documented in perlrun.

2. Use constant folding to remove the checks at runtime. In the following, notice how the debug code is removed when (and only when) a constant expression is used in the if:

>perl -MO=Deparse -e "$DEBUG = 0; print(qq{debug\n}) if $DEBUG; print(
+qq{end\n})"
$DEBUG = 0;
print "debug\n" if $DEBUG;
print "end\n";
-e syntax OK

>perl -MO=Deparse -e "use constant DEBUG => 0; print(qq{debug\n}) if D
+EBUG; print(qq{end\n})"
use constant ('DEBUG', 0);
'???';
print "end\n";
-e syntax OK
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