you are looking for assertions, unfortunatelly they only work on the unstable versions of perl. assertions::compat is also available but it requires you to test constants explicitelly as ...

dbg and expensive_test();
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Another similar module is ctflags (that stands for "compile time flags"), it is overly complex with too many options... but for what you need:

In some place, at the beggining of your script, include this code to set flags from an environment variable:

use ctflags::parse ns => "myapp:debug", env=>'MYAPPDEBUG';
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then in every module where you want to conditionally execute something add:

use ctflags prefix=>'dbg_', 'myapp:debug:*';

dbg_a and print "a flag is on\n";
dbg_b and print "b flag is on\n";
dbg_c and print "c flag is on\n";
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Finally, if you call your script as

 $ MYAPPDEBUG=ac perl myapp.pl
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it will output

a flag is on
c flag is on
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