Perl - Hash and the => operator - perl

Question

So Im learning Perl and got to the chapter about hashes. I understand that the '=>' operator is an alias to the comma operator. But when I try to make a value undef I get the warning
Use of uninitialized value $last_name{"dino"} in concatenation (.) or string at ./learning.pl line 18.
use warnings;
use strict;
my %last_name = (
fred => 'flintston',
dino => undef,
barney => 'rubble',
betty => 'rubble',
);
my $key;
my $value;
if(%last_name) {
foreach $key (sort keys %last_name) {
print("$key => $last_name{$key}\n");
}
}
But when I change the hash line to:
my %last_name = (
fred => 'flintston',
dino => undef =>
barney => 'rubble',
betty => 'rubble',
);
It works just fine and returns the value as undef. All I did was replace the comma operator separating the key/value with the '=>' operator.
Why does the later work but not the former if the two operators are supposed to be the same?

Answer 1

They are not exactly the same. The "fat comma" does one more thing: it quotes the left hand side operand when it's a bareword. Therefore,
undef, 1
is equivalent to
undef(), 1
while
undef => 1
is equivalent to
'undef', 1
See perlop:
The => operator is a synonym for the comma except that it causes a word on its left to be interpreted as a string if it begins with a letter or underscore and is composed only of letters, digits and underscores.

Answer 2

Choroba has already answered, but I think it can be useful to expand on this a little.
In the first case, when you get a warning, it's because you are printing an undef value: the value of the key 'dino' is undefined. Note that when you set the values of the hash you don't get any warning, because giving an undefined value to a key is perfectly normal. The warning shows up only when you try to use this undefined value; it would be better to check it before you use it, like this:
if (defined $last_name{$key}) {
print("$key => $last_name{$key}\n");
}
This way you can avoid the warning.
The interesting part comes in your second case. To understand what happens, you can run this code at the end:
if (defined $last_name{'dino'}) {
print "last_name{'dino'} is defined, and its value is $last_name{'dino'}\n";
if ($last_name{'dino'} =~ /^[u][n][d][e][f]$/) {
print "It matches, which means that the value of last_name{'dino'} " .
"is not undefined, it's the string 'undef'\n";
}
}
and both if are true, which indicates that the value for $last_name{'dino'} is defined, and it's the string 'undef'. Of course it's confusing, but it is defined. Therefore, you don't get any warning.
All of this is a way to see what is happening. To understand why, please check choroba's answer.