(1)    For example,

sub sum {
 my $result = 0;
 foreach (@_) {
  $result += (ref $_ eq 'ARRAY') ? sum(@{$_}) : $_;
 }
 $result;
}

(2)    For example,

use strict;
# t1aqb.pl

@ARGV < 1 && die "usage t1aqb.pl inputfile";
my $infile = $ARGV[0];
open DATA, "<$infile" || die "Can't open file $infile.";
my %majors = ();

while ($_ = <DATA>) {
 chomp;
 # Name starts at column 24.  Extra . before and
 # after (.{20}) removes leading and trailing " of
 # names.
 my ($major, $name) = /^(.{4}).{20}(.*).$/;
 push @{$majors{$major}}, $name;
}
close DATA;

# print result.
foreach (sort keys %majors) {
 print "$_:\n";
 foreach (sort @{$majors{$_}}) {
  print "  $_\n";
 }
}
exit 0;

(3)    (a)    baa

(b)    http://dcm.uhcl.edu/yue/q3.pl?user=Bun+Yue&submit=submit

(4)    For example,

sub processURL {
 my $url = shift;

 # Use LWP::UserAgent to get the page.
 my $ua = new LWP::UserAgent;
 my $response = $ua->request(new HTTP::Request('GET', $url));

 if ($response->is_error()) {
  print "<h2>Error in URL</h2>\n",
     "Sorry, your $url is erronous.";
 }
 else {
  # Append to file.
  if (open OUT, ">>$filename") {
   print OUT "$url\n";
   print "<h2>URL Added</h2>\n",
      "Your URL $url has been added.  Thanks.\n";
   close OUT;
  }
  else {
   print "<h2>URL not added</h2>\n",
      "Sorry, there is a file problem. Your URL has not bee added.";
  }
 }
} # process URL.

# print the form to solicit URL.
sub showForm {
 print <<_URL_FORM;
<form>
Enter your URL to be listed:
<br>
<input type="text" name="$URL_PARAM" size="40" maxlength="200">
<p>
<input type="submit" value="submit URL!">
</form>
_URL_FORM
} # showForm