sub foo {
    return length($_[0]);
}

##</code><code>##

sub dot_to_underscore {
    $_[0] =~ s/\./_/g;
}

my $string = 'hello.world';
print "$string\n";

dot_to_underscore($string);
print "$string\n";

##</code><code>##

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.
}