This is because date prints a newline, and the backquotes catch it, so the newlines are in the string $yr and $mm.

Date has to print a newline, as otherwise the output would get into the same line as your prompt if you call it from the shell (this is like this only on unix systems, dos prints a newline before the prompt).

To correct it, remove the newlines from the strings with the command

chomp ($yr, $mm, $dd);
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Update: Five identical answers to the question, wow!