The reason your multiChomp fails is because you are setting @out from $_, instead of from @_; it is the latter that holds the arguments to the function.

Just a couple of observations. One is that you don't need the loop in multiChomp; you can achieve the exact same results with chomp @out. Also, I don't see the utility of chomping the entire array and then returning only the first chomped element, which is what multiChomp does in scalar context. A more useful thing would be to return the concatenation of the chomped elements. Taken all this together, I would rewrite multiChomp like this:

sub multiChomp {
  my @out = @_;
  chomp @out;
  return wantarray ? @out : join '', @out;
}
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Or you could do this:

( my $text = do { local $/, @ARGV = 'foo.txt'; <> } ) =~ s/\n+//g;
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