At this point it's mostly a matter of curiousity but an interesting anomaly within either the eval function or the Perl regex engine.
It's not. It's just that you're trying to match a backslash, for which you need a double backslash in the regex. But to achieve that, you need to type 4 backslashes in a singlequotish string. It's what's in the string that matters for the regex engine, not what's in your source code.
A backslash only disappears in such a string, if it's in front of a backlslash or a string delimiter (here "'").
