Apparently those processes are not zombies. You'll need to kill the tcpdump process when the alarm goes off, and in order to do that you'll need to know its pid.

Actually, I think I'd use a different approach: 

my $pid = open(T, "tcpdump ...|");
eval {
  while (1) {
     $SIG{ALRM} = sub { die "time to stop" }
     alarm(20);
     my $line = <T>; # read a line from tcpdump
     alarm(0);
     last unless defined($line);
     # process line
  }
};
kill 9, $pid;
close(T);

[download]
Or, you can use IO::Select: 
my $pid = open my $T, "tcpdump ...|";
my $sel = new IO::Select($T);
my $buf;
while ($sel->can_read($T, 20)) {
  read($T, $buf, 1024, length($buf)) or last;
}
kill 9, $pid;
close($T);
# $buf contains output from tcpdump

[download]

In reply to Re^3: Eval leaving zombies when dying with alarm by pc88mxer
in thread Eval leaving zombies when dying with alarm by yogsothoth

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.