#!/usr/bin/perl

# https://perlmonks.org/?node_id=1226054

use strict;
use warnings;
use Tk;
$SIG{__WARN__} = sub { die @_ };

my %mults = (
    A => "a",
    B => "b",
    '' => '',
);
my %Inputs;
my %Telltale;

sub inline_validate_section {
    $Inputs{Section} or return 1;
    $Telltale{Section} or return 1;
    my $entry = uc(shift);
    print "$entry\n";
    if(defined($mults{$entry}) || !$entry){
        $Inputs{Section}->configure(-background => 'lightgrey');
        $Telltale{Section}->configure(-text => "$mults{$entry}");
        #$Telltale{Section}->update();
    } else {
        $Inputs{Section}->configure(-background => 'red');
        $Telltale{Section}->configure(-text => "nope");
        #$Telltale{Section}->update();
    }
    return 1;
}

 my $main = MainWindow->new();
 $main->geometry('400x100');

 # two frames and adjuster
 my $main_frame = $main->Frame; # right

  # pack frames
 $main_frame->pack(qw/-side right -fill both -expand l/);

     my $vn = "Section";

  $main_frame->Label(-text => $vn, -width => 15)->pack(qw/-side left/);
    $Inputs{$vn} = $main_frame->Entry(-validate        => 'key',
                    -validatecommand => [\&inline_validate_section],
                    -textvariable => \$vn)->pack(qw/-side left -padx 10 -pady 5 -fill x/);

  $Telltale{$vn} = $main_frame->Label(-text => "nope", -width => 25)->pack(qw/-side left/);

 MainLoop();