My grateful thanks to you all, particularly Rolf for your suggestions and prodding, and haukex for confirming the bug in Perl v5.26.*.

haukex, what did you search to find the bug references you cited?

BillKSmith, perltidy is new to me. But I tried it on my sample code; it did nothing with it (short of changing the whitespace). It didn't flag any issue with it.

Meanwhile, I've punted. The actual code was this:

  s{\s(")?(\w+):(\w)((([^"\n]*)")|([^\s\n]*)([\s\n]))}
   { " <a>$2:$3" . ( ($1 && $5) ? "$6</a>" : "$7</a>$8" ) }eg;
[download]

That finds phrases like 'source:tag' and 'source:some tag' and wraps each in HTML anchor tags. I rewrote it like this:

  my $p = qr{\s(")?(\w+):(\w)((([^"\n]*)")|([^\s\n]*)([\s\n]))};
  s|$p| " <a>$2:$3" . ( ($1 && $5) ? "$6</a>" : "$7</a>$8" ) |eg;
[download]

So the full search-and-replace operation fits fine on one line.

Thanks again. Enjoy what's left of your holiday, wherever you are.

--Lee