maybe you could try swapping hello and hi?

$string = str_replace("Hello", "Hi", "Hello My Name Is Elfyn");
[download]

Besides, if there are strange chars in $find ( \, +, *, (, [ or the like), better write:
$what =~ s/\Q$find/$replace/;
Otherwise, you may get nasty errors

Best regards,
perl -le "s==*F=e=>y~\*martinF~stronat~=>s~[^\w]~~g=>chop,print"