It's not an empty string, but a dualvar that has zero is the numeric slot and an empty string in the string slot. 
$ perl -wle'print ""'


$ perl -wle'print 0+""'
Argument "" isn't numeric in addition (+) at -e line 1.
0

$ perl -wle'print !1'


$ perl -wle'print 0+!1'
0

[download]

Anyway the wording "it returns a boolean value" is somehow misleading ...

It does return something that's a boolean value (true when there's a match, false when there's no match). It also returns ...

In reply to Re^3: Booleans from flip-flops by ikegami
in thread Booleans from flip-flops by LanX

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