substr 4th value generates bad code

[snoopyjc]

substr takes a 4th argument - a replacement. If you specify it, you get a #FAILTRAN error. Per the documentation;

my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from";    # climbed
# $s is now "The black cat jumped from the green tree"