"I tried those suggestions, but I still always run into the same error (example output provided below)."

It looks like you've hit the 64K limit: your output is truncated. Where you previously had

... -lpthread -ldl lmm -lutil -lc
[download]

followed by an error; you now have

... -lpthread -ldl -lm -lutil -lc
[download]

with no error. And no subsequent errors to the end of what's posted.

"I don't know whether it's perhaps something to do with the new 'System Integrity Protection' feature of MacOS limiting access etc?"

That shouldn't be an issue. I investigated SIP before upgrading to whatever version had that (10.10.x?). I have the default SIP configuration; I haven't changed anything associated with it; Perl installations work fine.

"In any case, I've switched to using Macports and things seem to be working okay. I'm using a local version of Perl in /opt/local/bin/ instead of the system Perl."

At least you now have a working Perl that isn't the System Perl.