A bit more digestible explanation (imo): 
$ perl -e 'printf "$_ => %b\n", ord($_) for qw( A B C @ )'
A => 1000001
B => 1000010
C => 1000011
@ => 1000000

[download]
so, "A" & "B" (0b1000001 & 0b1000010) yields "@" (0b1000000), "A" & "C" (0b1000001 & 0b1000011) yields "A" (0b1000001), "B" & "C" yields "B".

So, for example: 

"ABC" & "ABA" => "ABA"
"ABB" & "ABA" => "AB@"

"ABC" & "AB"  => "AB"
# perlop: "the & op acts as though the longer operand
# were truncated to the length of the shorter"
# so the strings don't have to be equal length
# for that to work

[download]

In reply to Re^2: Tri state string compare? (Solved! see update2) by Anonymous Monk
in thread Tri state string compare? (Solved! see update2) by BrowserUk

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