G'day haukex,

What you've described here, with respect to local, is pretty much how I've always understood this to work: no dark arts or black magic; effectively, just a simple pushing and popping of a stack.

In a subsequent reply, you referenced "perlsub: Temporary Values via local()" and then wrote: "Without some reassurance from the docs, one might worry that the temporary variables might somehow "go away" when the scope exits ...". The following code tracks %h through a do block (similar to your code); in addition, do contains an anonymous block which creates new temporary values. It also tracks the lexical $r: declared but uninitialised before do; assigned a value within the anonymous block; and shown to retain that value after do.

#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dump;

my $r;

print "\$r UNDEF\t"; dd $r;

our %h = (z => 26);

print "%h BEFORE do\t"; dd \%h;

my $x = do {
    print "PRE local\t"; dd \%h;

    local %h;

    print "POST local\t"; dd \%h;

    %h = (a => 1);

    print "POST assign\t"; dd \%h;

    print "%h BEFORE anon\t"; dd \%h;

    {
        print "PRE local2\t"; dd \%h;

        local %h;

        print "POST local2\t"; dd \%h;

        %h = (b => 2);

        print "POST assign2\t"; dd \%h;

        $r = \%h;

        print "\$r ASSIGNED\t"; dd $r;
    }

    print "%h AFTER anon\t"; dd \%h; 

    \%h;
};

print "%h AFTER do\t"; dd \%h;
print "\$x AFTER do\t"; dd $x;
print "\$r FINAL\t"; dd $r;
[download]

Output:

$r UNDEF        undef
%h BEFORE do    { z => 26 }
PRE local       { z => 26 }
POST local      {}
POST assign     { a => 1 }
%h BEFORE anon  { a => 1 }
PRE local2      { a => 1 }
POST local2     {}
POST assign2    { b => 2 }
$r ASSIGNED     { b => 2 }
%h AFTER anon   { a => 1 }
%h AFTER do     { z => 26 }
$x AFTER do     { a => 1 }
$r FINAL        { b => 2 }
[download]

Hopefully, that's a bit more reassuring. :-)