@ARGV is an array of strings. Bitwise OR or AND of strings in Perl results in a byte-by-byte OR or AND. Compare:

$ perl -e "print 1 & 63, qq(\n)"
1
$ perl -e "print q(1) & q(63), qq(\n)"
0
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As an integer bitwise AND, the result is 1. As a string bitwise AND, the result is 0. Why? As a string, ord("1") & ord("6") yields ord("0"). To try something cool, try:

$ perl -e "print q(a) & q(ab), qq(\n)"
a
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Similarily, ord("a") & ord("a") yields ord("a"). (The characters on the end don't count for bitwise AND since the strings have a different length, and Perl doesn't explicitly include the "\0" characters at the end)

Any time you have a scalar that may be in string context that you need to manipulate as a number, consider using a common expression such as '0+$scalar' to force it to have a numeric representation. @ARGV appears to be just such a case.