$ time perl -e '$a = "a" x 20_000; $a = substr($a, 1) while length($a);'
user    0m0.350s

$ time perl -e '$a = "a" x 40_000; $a = substr($a, 1) while length($a);'
user    0m0.791s

$ time perl -e '$a = "a" x 80_000; $a = substr($a, 1) while length($a);'
user    0m2.683s