This will work:

my $samesign, $first, $second;
if (($first * $second) > 0) {
    $samesign = 'true';
}
elsif (($first * $second) < 0) { {
    $samesign = 'false';
}
else {
    $samesign = undef;
}
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The else condition means that one of the numbers is zero, and sifgn doesn't mean anything then.