Just a little less work gets you a crude GUI version:

use strict;
use warnings;
use Tk;

my $obj = bless {}; # Create an object to make passing parameters easi
+er and to
    # maintain state. Generally considered better than using global va
+riables.

if (open my $tempIn, '<', "DB.txt") {
    while (<$tempIn>) {
        chomp;
        my ($name, $number) = split ':';

        next unless defined $number; #Skip empty or corrupt lines

        $obj->{phoneBook}{$name} = $number;
    }
    close $tempIn;
}

$obj->{main} = MainWindow->new (-title => 'My Simple Phone Book');
$obj->{main}->Button (-text => 'List', -command => [\&doList, $obj])->
+pack ();
$obj->{main}->Label (-text => "Name:")->pack ();
$obj->{main}->Entry (-textvariable => \$obj->{name})->pack ();
$obj->{main}->Button (-text => 'Del', -command => [\&doDel, $obj])->pa
+ck ();
$obj->{main}->Label (-text => "Number:")->pack ();
$obj->{main}->Entry (-textvariable => \$obj->{number})->pack ();
$obj->{main}->Button (-text => 'Add', -command => [\&doAdd, $obj])->pa
+ck ();
$obj->{main}->Button (-text => 'Exit', -command => [\&doExit, $obj])->
+pack ();

MainLoop ();


sub doAdd {
    my ($self) = @_;

    $self->{phoneBook}{$self->{name}} = $self->{number};
}


sub doExit {
    my ($self) = @_;

    open my $out, '>', "DB.txt" or die "Sorry Cant Open File: $!";
    for my $name (sort keys %{$self->{phoneBook}}) {
        print $out "$name:$self->{phoneBook}{$name}\n";
    }
    close $out;
    exit;
}


sub doDel {
    my ($self) = @_;

    delete $self->{phoneBook}{$self->{name}};
}


sub doList {
    my ($self) = @_;
    my $msg;

    for my $name (sort keys %{$self->{phoneBook}}) {
        $msg .= sprintf "%-20s %20s\n", $name . ':', $self->{phoneBook
+}{$name};
    }

    $self->{main}->messageBox (
        -icon    => 'info',
        -message => $msg,
        -title   => 'Phone numbers',
        -type    => 'Ok',
    );
}
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Note that the data is maintained in memory until the program exits.