I don't want to open a debate about when 'our' should or should not be used.

If wishes were horses, beggars would ride...

I guess I'm pretty baffled by this request...our does not do what my does in general, and for pre-5.6.0, the appropriate replacement for our is use vars. Why do you not want to just use vars for every variable you are using our for now? Possibly combined with a dummy our subroutine so you don't have to remove them (untested):

BEGIN { *our = sub { wantarray ? @_ : $_[$#_] } if $] < 5.006 }
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(May not parse exactly as our() does in every circumstance.)