The lexical variable $pid is evaluated and the result is then discarded. The variable is not tie-d to anything, so there can be no side effect. Don't you see a warning from this (although you don't use warnings; in your sample code)? All output from the $connection->rpc() function call is assigned to @resp. What's the point of having $pid in the statement? A typo or a thinko is strongly suggested.

>perl -wMstrict -le
"my $pid  = 'foo';
 my @resp = qw{ a b c };
 sub S { return qw{ FOO bar quux } }
 $pid, @resp = S();
 print qq{$pid @resp};
"
Useless use of private variable in void context at -e line 1.
foo FOO bar quux
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