Of course, sub someSub{} should run "quickly" or the GUI will "freeze".

Update: I re-read the question and perhaps I made an incorrect about this being a Tk GUI? If so, the OP needs to clarify.

Update with Code for a GUI:

use strict;
use warnings;
use Tk;

# quick hack
# reports the last function button pressed 
# every 3 seconds
# Doesn't start reporting until first button is pressed.

my @buttons = (["function1", \&func1],
               ["functoin2", \&func2],
               ["function3", \&func3],);

my $last_function_executed =0;

sub func1{
   print "this is function 1\n";
   $last_function_executed = 1;
}

sub func2{
   print "this is function 2\n";
   $last_function_executed = 2;
}

sub func3{
   print "this is function 3\n";
   $last_function_executed = 3;
}

sub report_last_func{
   return unless $last_function_executed; # no button pressed yet!
   
   print "most recent operation was: $last_function_executed\n";
}

my $MW = MainWindow->new();

foreach my $button_ref (@buttons)  # create buttons
{
    my ($text,$subref) = @$button_ref;
        
    $MW->Button(-text=> $text,
                -command => $subref)-> pack;
}

$MW->repeat(3*1000, \&report_last_func);

MainLoop;
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