Put my on the loop variables:

foreach my $oldkey (keys %fdat) {
    if ( (my $newkey = $oldkey) =~ s/^redirect_//) {
        $fdat{$newkey} = $fdat{$oldkey};
    }
}
[download]

That way you know you won't accidentally conflict with any globals of the same name. And it will pass the handy-dandy spell-checker known as strict.