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

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

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

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-wri
+te.
    return $string;      # No copy under 5.20+ until the *caller* modi
+fies the return string. But yes, the caller will get a copy pre-5.20.
}