This is what C programmers call 'undefined behaviour'. There is no point to try to explain why those regexes do what they do (whatever that is).

perlre:

There is a special form of this construct (look-behind), called "\K" (available since Perl 5.10.0), which causes the regex engine to "keep" everything it had matched prior to the "\K" and not include it in $& ($MATCH). This effectively provides variable-length look-behind. The use of "\K" inside of another look-around assertion is allowed, but the behaviour is currently not well defined.

perlvar:

In Perl v5.18 and earlier, it (${^MATCH}) is only guaranteed to return a defined value when the pattern was compiled or executed with the "/p" modifier. In Perl v5.20, the "/p" modifier does nothing, so "${^MATCH}" does the same thing as $MATCH.
The OP is using 5.010.

In reply to Re: Question on Regular Expression by Anonymous Monk
in thread Question on Regular Expression by sjain

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