Thanks for the feedback. I think I managed to lock a separate private variable of the object and now can use a socket inside the object that is not shared and therefore does not cause a conflict.

This is a fully working perl sample script:

#!/usr/bin/perl

use strict;
use warnings;
use threads;
use threads::shared;

{
    package Foo;

    use threads::shared;
    use constant INDENT    => "\t\t\t\t\t\t\t\t";

    sub new {
        my $class = shift;

        my %this;
        $this{_socket} = shift;
        $this{_sendBuffer} = [];

# TBD: Why does this not work?
#         my %this = {
#             _socket            => shift,
#             _sendBuffer        => [],
#         };

        share ($this{_sendBuffer});    # make this a shared variable t
+o allow main thread to lock & manipulate this

        return bless \%this, $class;
    }

    sub serve {
        my $this = shift;

        {    # scope for lock on $Foo->{_sendBuffer}
            print INDENT . "object thread: 1st lock attempt\n";
            lock ($this->{_sendBuffer});
            print INDENT . "object thread: 1st lock acquired...";

            push $this->{_sendBuffer}, "datagram " . scalar (@{$this->
+{_sendBuffer}});
             sleep (1);

             print "    and released!\n";    # pretend to release lock
+ before going out of scope
        }    # release lock

        sleep (1);

        {    # scope for lock on $Foo->{_sendBuffer}
            print INDENT . "object thread: 2nd lock attempt\n";
            lock ($this->{_sendBuffer});
            print INDENT . "object thread: 2nd lock acquired...";

            push $this->{_sendBuffer}, "datagram " . scalar (@{$this->
+{_sendBuffer}});
             sleep (1);

            print "    and released!\n";    # pretend to release lock 
+before going out of scope
        }    # release lock
    }
}

my $foo = new Foo();

my $thr1 = threads->create(sub { $foo->serve() });

{    # scope for lock on $foo
    print "main thread: 1st lock attempt\n";
    lock ($foo->{_sendBuffer});
    print "main thread: 1st lock acquired...\n";

    print "    foo send buffer size: " . scalar (@{$foo->{_sendBuffer}
+}) . "\n";
    sleep (1);

    print "main thread: 1st lock released!\n";
}    # release lock

sleep (1);

{    # scope for lock on $foo
    print "main thread: 2nd lock attempt\n";
    lock ($foo->{_sendBuffer});
    print "main thread: 2nd lock acquired...\n";

    print "    foo send buffer size: " . scalar (@{$foo->{_sendBuffer}
+}) . "\n";
    sleep (1);

    print "main thread: 2nd lock released!\n";
}    # release lock

$thr1->join();        # for some reason, this calls the destructor of 
+foo (if present): so the thread obtained a copy of foo to work with?

print "main thread: object thread joined\n\n";

my $index = 0;
foreach my $item (@{$foo->{_sendBuffer}}) {    # loop through contents
+ of sendBuffer
      print "foo->{_sendBuffer[$index]}: $item\n";    # output sendBuf
+fer contents
      $index++;
}

undef ($foo);    # destroy object
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I know, I am not really using a socket here - this sample was designed to find a way to use a "mutex" inside an object without locking the whole object. This is accomplished, and from my larger project I can now use a socket in the object.