Perl: Why is it slower to declare (my) variables inside a loop?

Question

What\'s the difference, from the interpreter\'s POV, between the following the following programs:

#!/usr/bin/perl -w

use strict;

for (1..10000000) {
    m         


        

Answer 1

Since the example programs you have given do not really do anything it is hard to give you a specific reason why one type of declaration would be better than the other. As many other posters have pointed out, declaring the variable in the loop creates a new variable each time. In your examples that creation is redundant, but consider the following examples using closures.

my @closures;
my $jimmy;

for (1 .. 10) {
    $jimmy = $_** 2;
    push @closures, sub {print "$jimmy\n"};
}

and this one:

my @closures;

for (1 .. 10) {
    my $jimmy = $_** 2;
    push @closures, sub {print "$jimmy\n"};
}

In each case the code builds up a series of code references, but in the first example since all the code refs refer to the same $jimmy each one will print 100 when called. In the second example each code ref will print a different number (1, 4, 9, 16, 25, ...)

So in this case the time difference does not really matter since the two blocks of code do very different things.