Here's a nice simple example of when you might need to use it:

$ perl -e 'print (2+3)/5'
5
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How on earth does that print 5? Because it evaluates as "print the result of 2 + 3, then divide print's return value (which is 1 if print worked) by 5, and then do nothing with the result". This is obvious if you stick another print in there:

$ perl -e 'print print (2+3)/5'
50.2
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which is parsed as:

print((print(2+3))/5)

And just to close the logic loop (++DrHyde and lidden, btw), this leading plus helps the parser evaluate all those numbers into a single value before printing.

$ perl -e 'print +(2+3)/5'
1
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So I assume the author of the code the OP was reading thought there might be some ambiguity and wanted to help the parse out with that return statement.

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I humbly seek wisdom.