perl -e "chomp(@F=<>); print join',',@F" file
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?
Or if you're not worried about possibly having a comma after the last line:

perl -pe "tr/\n/,/" file
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- or do it all with command line args and have no program:

perl -l0054pe '' file
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If you don't want a trailing comma:

perl -pe "eof or tr/\n/,/" file
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