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use strict;
use warnings;
...
}                

say +( $str =~ tr/1/1/ ) / $len;

$ perl 171126.pl 
0.4988
0.00312

$ perl -v

This is perl 5, version 26, subversion 0 (v5.26.0) built for x86_64-li
+nux-thread-multi
(with 1 registered patch, see perl -V for more detail)