perlre states:
As a workaround for this problem, Perl 5.10.0 introduces ${^PREMATCH} , ${^MATCH} and ${^POSTMATCH} , which are equivalent to $` , $& and $' , except that they are only guaranteed to be defined after a successful match that was executed with the /p (preserve) modifier.

Update: The problem you have documented is a bug in releases prior to 5.18.See Re: ${^MATCH} sometimes fails in target of substitution?.

Before 5.18 it appears that ${^MATCH} may only be available AFTER the match/replace. Adding print $(^MATCH} to your both example prints 'bcd' e.g. 

 perl -e '$x = "abcde";             $x =~ s/b../==${^MATCH}==/p;  prin
+t "$x Match:${^MATCH}\n"'
a====e Match:bcd
perl -e '$x = "abcde"; $sub="b.."; $x =~ s/$sub/==${^MATCH}==/p; print
+ "$x Match:${^MATCH}\n"'
a==bcd==e Match:bcd

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If you spot any bugs in my solutions, it's because I've deliberately left them in as an exercise for the reader! :-)

In reply to Re: ${^MATCH} sometimes fails in target of substitution? by space_monk
in thread ${^MATCH} sometimes fails in target of substitution? by raygun

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