Good job.

You forgot to accomodate the /g on the regexp. I'd prefer to write this as a s///g but here's your version, reconstituted for the /g feature.

while ( ( my $index = index substr( $_, pos() ), $find),
          $index != -1 )
{
    substr $_, $index, length $find, $replace;
    pos() = $index + length $find;
}
[download]

while ( ( my $index = index $_, $find),
          $index != -1 )
{
    substr $_, $index, length $find, $replace;
}
[download]