use strict; use warnings;
my $str="6123457";
while ($str =~ /(.)/g){
    last if pos($str) >= length($str);
    my $p=pos($str);    #backup pos beause overwrite $str afterwards
    my $l=substr($str, pos($str) -1 ,1);
    my $r=substr($str, pos($str),1);
    substr($str, pos($str)-1, 2)
        = ($r >= $l) ? $l.$r : $r.$l ; #replace with lvalue of substr
    print "str=$str\n";
    pos($str)=$p;
}
print "result=$str\n";