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s/\R(?!\z)//gm

$ perl -E 'my @x = ("a\nb\nc\n", "a\nb\nc", "abc\n", "abc");  for (0 .
+. $#x) { say "$_=|$x[$_]|"; $x[$_] =~ s/\R(?!\z)//gm; say "$_=|$x[$_]
+|" }'
0=|a
...
|
3=|abc|
3=|abc|