while(chomp (my $line = <DATA>)){
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No it still works with chomp

To understand why this works, take a look at the return value for chomp. Hint: it's not the string with newlines missing.

The problem with while (my $line = <DATA>) {} is real, it's just obscure. As hinted, it only happens when you get a line with a false value and no newline. Sometimes this happens at the end of a file. Sometimes it happens for other reasons. It usually doesn't cause any actual problems, but certainly is something to look out for.

The problem with while (my $line = <DATA>) {} is real, it's just obscure

ARRRRGH! :-)

As has already been pointed out this hasn't been a problem for some time. Perl automatically adds the defined for you, as you can easily see by running:

a% perl -MO=Deparse -e 'while (my $line = <DATA>) {}'  
while (defined(my $line = <DATA>)) {
    ();
}
__DATA__
-e syntax OK
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This hasn't been a problem for Perl for quite some time, and I wish people wouldn't keep saying it is ;-)

With this method you'll never be able to loop through the last line.