I'd just like to expand a little on grep's comments above, to explain why this script could be used as a spam portal.

print MAIL "To: $form{'usermail'}\n";

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Somebody could simply copy and paste an entire mailing list into the 'usermail' field and submit it.
The email would then be sent to everyone in that list. 
victim1@domain.com, victim2@domain.com, victim3@domain.com, etc...

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You get the picture.
This is made worse because the $form{'message'} is sent as the body of the email, meaning the (ab)user could enter their own adverts, urls etc into the 'message' field, so everyone would receive spam (possibly offensive) and it would appear to have been sent by you.

If you switch on taint mode 

#!/usr/bin/perl -wT

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Then perl will force you to run every user inputted data through a regex before you can send it outside of your program, such as to sendmail. 
my $username;
If ($form{username} =~ /^([\w.-]+)$/) {
  $username = $1;
}
else {
 die("invalid username");
}

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See the regex docs for help.
You would need to run every user input through a regex. The difficult one is validating the email address, have a search around the monastery for help with that.

In reply to Re: Insecurities in my scripting by fireartist
in thread Insecurities in my scripting by sulfericacid

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