So, the second line evaluates to true in the while-condition, but not in the following if-condition. Is there some kind of special magic? If yes, where is it documented?

the only location where i found it mentioned after a quick search is in perlvar $_:

$ARG
$_      The default input and pattern-searching space.
        The following pairs are equivalent:

                   while (<>) {...}    # equivalent only in while!
                   while (defined($_ = <>)) {...}
[download]

edit: and in perlsyn
edit: ah, it's in perlop, like betterworld mentioned. overlooked it.