You can edit the file in place using this:

syslog before the following command:
Oct  7 08:30:01 deneb sendmail[13183]: [ID 801593 mail.info] g97CU1r13
+183: from=
cmjohnso, size=252, class=0, nrcpts=1, msgid=<200210071230.g97CU1r1318
+3@deneb.us
lec.net>, relay=cmjohnso@localhost
Oct  7 08:30:01 deneb sendmail[13185]: [ID 801593 mail.info] g97CU1r13
+183: to=cm
johnso, ctladdr=cmjohnso (10052/1), delay=00:00:00, xdelay=00:00:00, m
+ailer=loca
l, pri=120252, relay=local, dsn=2.0.0, stat=Sent

% perl -i -p del.pl /var/log/syslog

syslog after the above command:
cmjohnso@deneb$ cat syslog
Oct
Oct
[download]

Here is a code example for del.pl:

#!/usr/bin/perl -w

use strict;

my @line = split(" ",$_);
print "$line[0]\n";
$_ = '';
[download]

Remember, when you are in the del.pl program you are only working on one line at a time, the 'perl -i -p <progrname> <file>' line reads each line of the file, in this case /var/log/syslog and applies the code in the program to each line, printing whatever you print in the code as well as the final content of $_. That is why I set $_ to '', so it did not print back to my file. It would have been just as east to set $_ to what I wanted to be printed to the file and not done the print myself.

You could write a program somewhat similar to this that evaluated each line and if it needs to be deleted from the file just set $_ to '' otherwise leave $_ alone and it will go back to your final file as it was when it entered the loop.

Here is the text from the "Perl Black Book" by Coriolis:

"Perl lets you make changes to files in-place--that is, make changes to the file directly, without having to explicitly read it in and write it out. To edit files in-place, you use the -i switch with Perl; this switch specifies that files processed with the <> construct to be edited in-place. ....... Note also the -p switch; this switch makes Perl use a while (<>) and print loop around your script to print the changed text back to the file."