regexp is challenging and fun, but not always the best solution, sometime it could be ugly and not maintainable.

In this case, first split on /, to get the prot part, then use the power of bitwise XOR like this (I choose not to use natural language to descibe, you should be able to understand what I am trying to say):

use strict;

my $str = "paat";
(my $cmp = $str  ^ "prot") =~ s/(.)/ord($1) ? "0" : "1"/eg;
print $cmp; 
#prot => 1111
#pro? => 1110
#pr?? => 1100
#p??? => 1000
#???? => 0000
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