Unless a package is reported as thread safe you must assume it is not thread safe. As such, you cannot use Tk outside your sub. The following works fine (no free to wrong pool errors). 
#!/usr/bin/perl -w

use strict;
use threads;

# Create main window
sub gui {
    require Tk;
    my $main = MainWindow->new;

# Add a Label and a Button to main window
    $main->Label(-text => 'Hello, world!')->pack;
    $main->Button(-text => 'Quit',
        -command => [ $main => 'destroy']
    )->pack;

# Spin the message loop
    Tk->MainLoop;
}

my $server_thread = threads->create("gui");

while (1) {
    sleep(1);
    print "cav\n";
}

[download]
Seeing how the above works, I believe Tk would be considered thread friendly. You may wish to consider using Thread::Use.

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In reply to Re: Making a TK GUI in a thread by PodMaster
in thread Making a TK GUI in a thread by cavalive

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