In the same vein I'd advise the OP to constrain the scope of his variables to make it easier to see where they are used and to avoid the bogus initialisations.

Always initialising variables seems like a good idea, but is only appropriate where the variable is being given a default value. If it is intended that the initial value will always be replaced, it is much better not to provide a bogus intial value. Perl's use warnings; "undef used" warning gives an immediate heads up that there is a path or condition in your code that you hadn't anticipated is you leave variables uninitialised until you have a sensible value for them.

Reworking the code again with that in mind and after fixing various issues that arise during testing you might end up with:

#!/usr/bin/perl
use strict;
use warnings;

my @nme;

# Input loop
while ((print "Enter the name: "), defined (my $input = <STDIN>)) {
    chomp $input;

    last if ! length $input; # Exit loop on empty line
    my ($fn, $ln) = split /\s+/, $input, 2;
    
    if (! defined $fn) {
        print "Blank name ignored\n";
        next;
    }
    
    $ln ||= '_missing_'; # Provide missing name
    print "$fn, $ln\n";
    push @nme, "$ln, $fn";
}   

# Print loop
print "$_\n" foreach sort @nme;
[download]

Note too the use of the comma operator in teh while loop expression to get the prompt output each time through the loop. Not a highly recommended technique, but can be less ugly and more cogent than many of the alternative ways of achieving the same effect.

Note too that the hash changed into an array. It's quite possible for two people to have the same last name. In fact it is even possible for people to have the same name. Names are tricky. ;)

Note too the use of the comma operator in the while loop expression to get the prompt output each time through the loop.Not a highly recommended technique, but can be less ugly and more cogent than many of the alternative ways of achieving the same effect.

On *nix I usually set up my prompt outside the while loop along with another string of returns and spaces to erase the prompt once I drop out of the loop. Like this

my $prompt = q{Enter name (<Ctrl-D> to exit) : };
my $dePrompt
   = qq{\r}
   . q{ } x (length($prompt) + 2)
   . qq{\r};
while (1)
{
    print $prompt;
    last if eof STDIN;
    chomp(my $response = <STDIN>);

    # Do something with $response here ...

}
print $dePrompt;
[download]

This has the advantage of cleaning up the last prompt displayed before returning to a shell prompt or some further output from the script. It does not work so well under MS Windows as it seems you have to do <Ctrl-Z><Enter> to signal EOF and that throws a line before you can erase the prompt. Thus, you may as well dispense with the $dePrompt part of it there.

I am not sure why you do defined (my $input = <STDIN>). Will it not always be defined, even with just hitting <Enter> as $input will always contain at least a newline? Perhaps I'm missing something but I would have thought you'd only have to worry about definedness after the chomp.

Cheers,

JohnGG