See perlop:

m/PATTERN/cgimosx
    (...)
    If the PATTERN evaluates to the empty string, the last successfull
+y matched 
    regular expression is used instead.
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Then, contemplate on what the effects of this (somewhat odd) feature are with your $$hash{"p1"} being empty...

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P.S.: you'd typically write $s =~ /.../; (i.e. use an explicit pattern delimiter) even though Binding Operators says

    If the right argument is an expression rather than a search patter
+n, substitution, 
    or transliteration, it is interpreted as a search pattern at run t
+ime.
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P.P.S.: also note that the dots in a pattern like /19.28.18.28/ are regex metacharacters, so maybe you want to use quotemeta  (or rather simply use eq string comparison...?)