Perl: why are these not equivalent?

Question

m/(?<=\n)(?=$pattern)/

vs

m/(?<=\n)(?=^$pattern)/

The semantic is I want to match the "empty string" between a new line character and a pattern.

However, in Perl, the top one does what I want while the 2nd one doesn't.

Could someone explain the intricacies at play here with the "^" meta character?

Thanks

Answer 1

^ defaults to matching the beginning of the string; if you want to match the beginning of a line, you need to use the /m flag or (?m:^).