I'm glad to see use utf8; was sufficient to resolve your problem.

Unrelated to your initial problem, I thought that I'd point out a few issues with:

open (DATA, "source.txt") || die "error opening file";
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• Use a lexical filehandle and the 3-argument form of open. That documentation has a number of examples and further discussion.
• DATA is a package variable and a particularly bad choice anyway. See "perldata: Special Literals".
• Hand-crafting die messages is tedious and easy to get wrong. "error opening file" gives no indication of the file that had the problem or what the problem was (for instance, non-existent file or insufficient privileges). Save yourself the effort of doing this task and let Perl report errors for you with the autodie pragma.