tr/0// == tr/1// and index $_, '10' < 0

##</code><code>##

my $hl = (1+length) >> 1;
tr/0// == $hl and tr/1// == $hl and index $_, '10' < 0

##</code><code>##

/^(0*)(1*)$/ and length $1 == length $2

##</code><code>##

tr/0// == tr/1//