Here is the simplified code that I'm using. The only interesting thing that happens is in the second to last line of code where I set $$x{foo}. If I use doit_local then $$x{path} becomes an undefined value, however if I use doit_my, $$x{path} retains its proper value.

#!/usr/bin/perl
use 5.008;
use strict;
use warnings;
use threads;
use threads::shared;
use Data::Dumper;

my %p1 :shared;
my %p2 :shared;

%p1 = (path => shift);
%p2 = (path => shift);

my @t;
push @t, threads->create(\&get_md5sums, \%p1);
push @t, threads->create(\&get_md5sums, \%p2);
$t[0]->join;
$t[1]->join;

print Dumper \%p1, \%p2;

sub doit_local {
  local $_ = shift;
  s/'/'\''/g;
  return $_;
}

sub doit_my {
  my $x = shift;
  $x =~ s/'/'\''/g;
  return $x;
}

sub get_md5sums {
  my $x = shift;
  print Dumper [$x, $$x{path}];
  $$x{foo} = join " ", map '"'.doit_local($_).'"', 'find', $$x{path}, 
+qw/-type f -exec md5sum {} ;/;
  return 1;
}
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Here is the output when I use doit_my. This is the desired output

[duelafn@jhegaala tmp]$ ./local_threads foo bar
$VAR1 = [
          {
            'path' => 'foo'
          },
          'foo'
        ];
$VAR1 = [
          {
            'path' => 'bar'
          },
          'bar'
        ];
$VAR1 = {
          'path' => 'foo',
          'foo' => '"find" "foo" "-type" "f" "-exec" "md5sum" "{}" ";"
+'
        };
$VAR2 = {
          'path' => 'bar',
          'foo' => '"find" "bar" "-type" "f" "-exec" "md5sum" "{}" ";"
+'
        };
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And this is what I get when I use doit_local. Before localizing $_ the shared hash has its correct values, however afterward, the path key is undefined.

[duelafn@jhegaala tmp]$ ./local_threads foo bar
$VAR1 = [
          {
            'path' => 'foo'
          },
          'foo'
        ];
Use of uninitialized value in substitution (s///) at ./local_threads l
+ine 25.
Use of uninitialized value in concatenation (.) or string at ./local_t
+hreads line 38.
$VAR1 = [
          {
            'path' => 'bar'
          },
          'bar'
        ];
Use of uninitialized value in substitution (s///) at ./local_threads l
+ine 25.
Use of uninitialized value in concatenation (.) or string at ./local_t
+hreads line 38.
$VAR1 = {
          'path' => undef,
          'foo' => '"find" "" "-type" "f" "-exec" "md5sum" "{}" ";"'
        };
$VAR2 = {
          'path' => undef,
          'foo' => '"find" "" "-type" "f" "-exec" "md5sum" "{}" ";"'
        };
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My perl version:

[duelafn@jhegaala tmp]$ perl --version

This is perl, v5.8.4 built for i386-linux-thread-multi
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