Chromatic gave an excellent answer, but to answer your original question, this should work:

print OUT map {"DR EMBL: ", join(';',(${$_}[1],${$_}[2])), "\n"} grep 
+{ ${$_}[0] eq "EMBL" } $entry->DRs->elements();
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This will add 3 elements to the list that is the return value of map:

1. The string "DR EMBL: " (using ${$_}[0] here isn't necessary since the rest of the elements except for EMBL have been filtered out using the grep)
2. A string created by joining the elements ${$_}[1] and ${$_}[2] with a semicolon.
3. And finally, a newline

I tried to follow the specifications that you wanted, but they weren't exactly clear... If you need something more, just ask.