$ perl -E 'BEGIN {our $debug = 1;} use if $debug // 0, warnings; say undef'
Use of uninitialized value in say at -e line 1.

##</code><code>##

$ debug=1 perl -E 'use if $ENV{debug} // 0, warnings; say undef'
Use of uninitialized value in say at -e line 1.