To elaborate a bit (it took me some experimenting to realize why cfreak is right), inside a
map the variable
$_ is magic. While each entry in the list is evaluated,
$_ is an alias to that entry. Assigning a new value to
$_ changes the value of the current list entry. Because you're calling
screwed from within
map without localizing
$_, it's being assigned to each successive line in the file, and then set to
undef to indicate end-of-file. When
screwed returns,
$_ is still set to
undef, so all entries in the list are changed to
undef.
You can verify this by changing screwed to:
sub screwed {
$_ = 'hi';
}
in which case you'll get:
before a
before b
after hi
after hi
