This is an external XS module that I am compiling.

Searching on that error gives me all sorts of doom and gloom about how this can lead to program malfunction, addressing memory by two different pointer types!

Not sure I believe it. Am tempted just to turn off strict-aliasing for that section. :)

This is an external XS module that I am compiling

Yes, but XS modules are normally compiled using a similar set of compiler flags to that used to compile perl itself. Which is why I'm surprised that -fno-strict-aliasing isn't being passed. Compiling XS code without that flag while using perl macros which are intended to be used with that flag, is asking for subtle optimiser bugs to be introduced into your compiled code.

Dave.

This is an external XS module that I am compiling

But if perl was built with -fno-strict-aliasing then one would expect that the compiler would not emit the warning when building an external XS module.
If you provide us with the output of perl -V and tell us where we can view the entire source of the XS module, then we can perhaps shed some light on the matter.

Cheers,
Rob

/usr/bin/perl /usr/share/perl5/vendor_perl/ExtUtils/xsubpp  -typemap /
+usr/share/perl5/ExtUtils/typemap -typemap typemap  Utils.xs > Utils.x
+sc && mv Utils.xsc Utils.c
Please specify prototyping behavior for Utils.xs (see perlxs manual)
gcc -c  -I/DBA/lua/5.2.1/include -Wall -Wno-strict-aliasing -O2 -g -pi
+pe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-stron
+g --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic
+   -DVERSION=\"1.0\" -DXS_VERSION=\"1.0\" -fPIC "-I/usr/lib64/perl5/C
+ORE"   Utils.c
Utils.xs: In function ‘perl_to_lua’:
Utils.xs:263:13: warning: dereferencing type-punned pointer will break
+ strict-aliasing rules [-Wstrict-aliasing]
             char *key = HePV (he, key_len);
             ^
Utils.xs:263:13: warning: dereferencing type-punned pointer will break
+ strict-aliasing rules [-Wstrict-aliasing]
Utils.xs:263:13: warning: dereferencing type-punned pointer will break
+ strict-aliasing rules [-Wstrict-aliasing]
Utils.xs:263:13: warning: dereferencing type-punned pointer will break
+ strict-aliasing rules [-Wstrict-aliasing]
Running Mkbootstrap for N2::Lua::Utils ()
[download]

It's 1000 lines of XS code. I'd have to trim it down a bit.

But in any case I think the answer is environmental, not in the code, as it compiles without warnings under Ubuntu 18.04, but WITH warnings under Oracle Linux Server 7.3.

Here's the full build output. And I see that -Wno-strict-aliasing is actually present in the GCC options! So why on earth are we still getting that warning?