That won't work if $pat is zero. Fix:

length($pat) && ($msg =~ s/$pat/$replace/mg);
[download]

I have to note this to try to scrape together a little dignity after having just posted a question on something in the man pages like a newbie coder :)

It's not well known, and it's easy to forget, because it's not a situation that's usually encountered. No dignity lost.

By the way compiling the pattern is not enough. I ran a test that showed $pat = qr//; /$pat/ is the same thing as m// even though the stringification of $pat is not an empty string.