I think what you were looking for is

$ perl -E 'BEGIN {our $debug = 1;} use if $debug // 0, warnings; say u
+ndef'
Use of uninitialized value in say at -e line 1.
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although I would be inclined to go with an environment variable instead, to avoid having to edit the source code:

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