I have RTFM'ed pack() and unpack(), but don't understand its use in this context. What is the "TEMPLATE" being used here?

unpack TEMPLATE,EXPR
unpack does the reverse of pack: it takes a string and expands it out 
+into a list of values. (In scalar context, it returns merely the firs
+t value produced.)
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This line in the manual I find obscure as well, though it seems it would help me if I understood it:

sub ordinal { unpack("c",$_[0]); } # same as ord()
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Could you explain unpack "U*"? What is the "U"? (something to do with Unicode? I listened to "Well you needn't" (angular piano music)last night in celebration of the post showing the table for converting with cp 1252