I guess Perl first creates the result of the operation, and then copies the value to the variable. In today's Perl, the result is created at compile time if both the string and the number are constant (not sure about your ancient version).
map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]