$ perl5.16.0 -wE'
no warnings "redefine";
sub FOO () { 23 }
sub FOO () { 42 }'
$
[download]

perldiag says "(S) A severe warning (enabled by default)." which is probably part of the problem. But it is still easy to get around in a couple of different ways:

#!/usr/bin/perl -w
BEGIN { $SIG{__WARN__} = sub {
    warn @_ if $_[0] !~ /^Constant subroutine/;
} }
sub FOO() { 23 }
sub FOO() { 42 }
[download]

Or

#!/usr/bin/perl -w
sub FOO() { 23 }
BEGIN { no warnings 'misc'; undef &FOO if ! @ARGV }
sub FOO() { 42 }
[download]

I threw in the 'if ! @ARGV' just so one can see the warning produced or suppressed by the same code.

- tye        

C:\test>perl -wE"no warnings 'redefine';sub FOO(){23};sub FOO(){42};sa
+y $];"
Constant subroutine FOO redefined at -e line 1.
5.014002
[download]

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

RIP Neil Armstrong

Not sure if it helps, but diagnostics shows:

Constant subroutine FOO redefined at -e line 1 (#1)
    (S) You redefined a subroutine which had previously been
    eligible for inlining.  See perlsub/"Constant Functions" for
    commentary and workarounds.
[download]

Leading to Constant Functions, which mentions a workaround.