$str =~ s/\\u([0-9a-fA-F]{4,6})/chr(hex($1))/eg;

It looks like Java unicode escapes. These always have exactly 4 digits as far as I've seen. \u20AC80 would mean €80, but your example would not see it that way :)

s/\\u([0-9A-Fa-f]{4})/chr hex $1/ge;

    or die "Can't open file `$file' for writing: $!";
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Backticks and apostrophes as balanced quotes are ugly, and not at all balanced, in just about any font.