Maybe I'm missing something, but I can't see where $_ gets set to anything useful (which is probably a good thing overall). Where you have $_ I think you need to have $file instead, since you have named it in the for declaration. Then also change the s/// to operate on $file:

$file =~ s/^append/aPPend/;
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If you omit the left hand side, the s/// will apply to $_, but as I mentioned, I don't think that's being set.

Phil