my $n = length $$strref;
# shift/rotate string and compare to original
my $diff = $$strref ^ substr( $$strref, -$i ).substr( $$strref, 0, -$i );
# number of differing characters between shifted string and original
my $ndiff = $diff =~ tr/\0//c; 
return 1 if $ndiff/$n < $tolerance