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';
}

[download]
in which case you'll get: 
before a
before b
after hi
after hi

In reply to Re: Re: opening a file destroys nulling entries in a list?!?! by sgifford
in thread opening a file destroys nulling entries in a list?!?! by matpalm

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.