sub dot_to_underscore {
    my $string = $_[0];  # No copy on 5.20+, but yes, copy under <5.20.
    $string =~ s/\./_/;  # Copy made under 5.20 because of copy-on-write.
    return $string;      # No copy under 5.20+ until the *caller* modifies the return string. But yes, the caller will get a copy pre-5.20.
}