Probably slower than what you're using, but easier to understand (I hope):

for(my $str = 'vaction.msg'; $str; $str = substr($str, 1, length($str)
+)) {
    $replace_on =~ s/$str//;
}
[download]

Ahh, the rarely seen (around here, anyway) three-element form of for. I've tested this loop and it produces the strings correctly. I don't know if this exactly matches your needs, but you should be able to modify it easily enough.

Update: Forgot that you need to escape the '.' before it gets put in a regex, which complicates things a bit.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated