note ikegami <ul> <li>For <c>for (X..Y)</c>, Perl uses a counting loop. <li>For <c>for (reverse constX..constY)</c>, Perl builds a flattened list at compile-time and iterates over it in reverse. <li>For <c>for (reverse X..Y)</c>, Perl builds a flattened list at run-time and iterates over it in reverse. </ul> <p>So far, I've found 6 different kinds of for loops in Perl. You can find them in [id://594208|an earlier node]. <c> #!/usr/bin/perl -- use strict; use warnings; use Benchmark qw(cmpthese); my $BIG = 100_000; cmpthese(-3, { c_f => \&c_f, c_r => \&c_r, p_fv => \&p_fc, p_fc => \&p_fv, p_rc => \&p_rc, p_rv => \&p_rv, }); sub c_f { for (my $i = 0; $i < $BIG; $i++ ) { 1 } } sub c_r { for (my $i = $BIG; $i-- > 0; ) { 1 } } sub p_fc { for my $i ( 0..100_000-1 ) { 1 } } sub p_fv { for my $i ( 0..$BIG-1 ) { 1 } } sub p_rc { for my $i ( reverse 0..100_000-1 ) { 1 } } sub p_rv { for my $i ( reverse 0..$BIG-1 ) { 1 } } __END__ Rate p_rv c_r c_f p_rc p_fv p_fc p_rv 28.5/s -- -37% -44% -59% -64% -65% Builds list and loops. c_r 45.0/s 58% -- -11% -35% -44% -44% c_f 50.7/s 78% 13% -- -27% -37% -37% p_rc 69.7/s 144% 55% 37% -- -13% -14% Loops over pre-built array. p_fv 80.0/s 180% 78% 58% 15% -- -1% Counting loop p_fc 80.7/s 183% 79% 59% 16% 1% -- Counting loop </c> <p>If you add "<c>(),</c>" to the front of any of the lists, you'll notice that p_* will become as slow as p_rv. 723825 723853