$ time perl -e '$a = "a" x 50_000; substr($a, 0, 1, "") while length($a);'
real	0m0.016s
user	0m0.012s
sys	0m0.008s
ysth@cat:~$ time perl -e '$a = "a" x 100_000; substr($a, 0, 1, "") while length($a);'
real	0m0.029s
user	0m0.024s
sys	0m0.004s
ysth@cat:~$ time perl -e '$a = "a" x 200_000; substr($a, 0, 1, "") while length($a);'
real	0m0.041s
user	0m0.036s
sys	0m0.004s