If you're replacing, there was a successful match. This is a bug.

Testing reveals this bug has been fixed:

$ perl -v | grep 'This is'
This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-li
+nux-thread-multi

$ perl -E'$_ = "abcde";             s/b../==${^MATCH}==/p;  say'
a====e

$ perl -E'$_ = "abcde"; $sub="b.."; s/$sub/==${^MATCH}==/p; say'
a==bcd==e
[download]

$ perl -v | grep 'This is'
This is perl 5, version 18, subversion 0 (v5.18.0) built for x86_64-li
+nux-thread-multi

$ perl -E'$_ = "abcde";             s/b../==${^MATCH}==/p;  say'
a==bcd==e

$ perl -E'$_ = "abcde"; $sub="b.."; s/$sub/==${^MATCH}==/p; say'
a==bcd==e
[download]