The oldest I have is 5.6.0, but it doesn't give a warning even when uncommenting the last line.

Changing no warnings; to use warnings; shows that the following creates a constant in 5.6.0 and 5.6.1, but not in 5.8.0: 

sub with_proto ()  { my $lex }

[download]
>c:\progs\perl560\bin\perl test.pl
Use of uninitialized value at test.pl line 12.
Constant subroutine with_proto redefined at test.pl line 12.
Subroutine with_proto redefined at test.pl line 16.
Subroutine with_proto redefined at test.pl line 20.
Subroutine with_proto redefined at test.pl line 25.
Subroutine with_proto redefined at test.pl line 29.
Subroutine with_proto redefined at test.pl line 36.
Subroutine with_proto redefined at test.pl line 41.
Subroutine with_proto redefined at test.pl line 45.
Subroutine with_proto redefined at test.pl line 55.
Constant subroutine with_proto redefined at test.pl line 58.

>c:\progs\perl561\bin\perl test.pl
Use of uninitialized value at test.pl line 12.
Constant subroutine with_proto redefined at test.pl line 12.
Subroutine with_proto redefined at test.pl line 16.
Subroutine with_proto redefined at test.pl line 20.
Subroutine with_proto redefined at test.pl line 25.
Subroutine with_proto redefined at test.pl line 29.
Subroutine with_proto redefined at test.pl line 36.
Subroutine with_proto redefined at test.pl line 41.
Subroutine with_proto redefined at test.pl line 45.
Subroutine with_proto redefined at test.pl line 55.
Constant subroutine with_proto redefined at test.pl line 58.

>c:\progs\perl580\bin\perl test.pl
Subroutine with_proto redefined at test.pl line 12.
Subroutine with_proto redefined at test.pl line 16.
Subroutine with_proto redefined at test.pl line 20.
Subroutine with_proto redefined at test.pl line 25.
Subroutine with_proto redefined at test.pl line 29.
Subroutine with_proto redefined at test.pl line 36.
Subroutine with_proto redefined at test.pl line 41.
Subroutine with_proto redefined at test.pl line 45.
Subroutine with_proto redefined at test.pl line 55.
Constant subroutine with_proto redefined at test.pl line 58.

[download]

Update: I identified the wrong one. Fixed.

In reply to Re^3: Inlining a "lexically-scoped scalar which has no other references" by ikegami
in thread Inlining a "lexically-scoped scalar which has no other references" by LanX

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