Shouldn't these situations be handled by garbage collection.

sub bla {
  my $arg = shift;
  my $big_string = $arg x 1000;
}
[download]

Perl will in general keep the memory for $big_string allocated and reserved, because then it doesn't need to allocate the memory again next time the sub is called.

Explicitly undef()ing or resizing variables before they go out of scope sometimes helps, though - on some systems, it might even free the memory back to the system.

Usually, you don't need to do this, exactly because the memory gets reused anyway. If your program grows a lot, it's more likely you're using an inefficient algorithm, or you're creating circular references somewhere.

So basicly if one has a large variable, which should be removed by scoping (not counting the speed usage but memory in should word), one should do manually. But if the large variable is within sub and that sub is used again (eg from a loop structure), then all the variables within it is automatically reused.

That's how I understand it, yes. Lexicals are re-used when iterating though a loop / subroutine note: recursive subroutine calls create a "stack" of lexicals - for obvious reasons - lookup goto &sub if you want to do tail recursion.

Ofcourse, if your program is structured well enough, you usually don't have to worry about it - what I mean is, if your variables go out of scope, you usually are going out of a sub or loop in which they will be used again.

The usual exception is some code that runs in stages, and you want to free as much memory as possible after you're done with one stage and enter the next, for instance, some code that needs to preprocess a lot of data in memory, write it out to a file and then go through the resulting file line by line - or something like that.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

Note that in your example, Perl will keep the memory for $big_string twice:

#!/usr/bin/perl

use strict;
use warnings;
use Readonly;

Readonly my $size => 10 * 1024 ** 2; # 10 Mb.

sub show_mem {
    system "grep ^VmSize /proc/$$/status";
}

sub gimme_big {
    my $size = shift;
    my $var  = 'x' x $size;
}

show_mem;
gimme_big $size;
show_mem;

__END__
VmSize:     3464 kB
VmSize:    23952 kB
[download]

For a 10Mb string, Perl allocates about 20Mb memory.

undefing the variable makes Perl allocate about 10Mb less:

#!/usr/bin/perl

use strict;
use warnings;
use Readonly;

Readonly my $size => 10 * 1024 ** 2; # 10 Mb.

sub show_mem {
    system "grep ^VmSize /proc/$$/status";
}

sub gimme_big {
    my $size = shift;
    my $var  = 'x' x $size;
    undef $var;
}

show_mem;
gimme_big $size;
show_mem;

__END__
VmSize:     3472 kB
VmSize:    13716 kB
[download]

So, how do we get rid of the extra 10Mb? By a careful use of string eval:

#!/usr/bin/perl

use strict;
use warnings;
use Readonly;

Readonly my $size => 10 * 1024 ** 2; # 10 Mb.

sub show_mem {
    system "grep ^VmSize /proc/$$/status";
}

sub gimme_big {
    my $size = shift;
    my $var  = eval "'x' x $size";
    undef $var;
}

show_mem;
gimme_big $size;
show_mem;

__END__
VmSize:     3468 kB
VmSize:     3468 kB
[download]

All of the variables in that loop will retain their memory between calls and reuse it. However, if you leave that loop, they will still retain that memory. Also, if you load one of them once with 10MB of data, it will stay at that size until perl exits unless you explicitly undef it when you're done with it.

I would probably help if you could show some code.
"What should it profit a man, if he should win a flame war, yet lose his cool?"