The answer to your first question can be found in perlop in the section Regexp Quote-Like Operators

If you use the /s modifier, then . matches any character, including newlines. So for example:

"a\nb" =~ m/a.*b/;    # no match
"a\nb" =~ m/a.*b/s;   # matches
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For handling TCP packets I'd rather use an existing module from CPAN.