Taint.pm is a module that helps ensure that a user doesn't pass your cgi script some input that can do bad things to your server. For instance, if you accepted some input and then did a system( $input ) where the input was 'rm -Rf /' then you could have problems, even if you were running http's as 'nobody' (OK, so this is a unix-centric answer ) I think it basically does string substitution to remove potentially hazardous characters:

WWW::Taint.pm
#
# Utility routines for untainting strings.  All three will accept
# a single scalar as argument, and return the same scalar back
# or undef if it flunks.  It's possible to call untaint() directly
# and supply your own regexp, but probably better to extend the
# package with your own method if you really need it.
#
# the empty string returns an empty string (meaningless but untainted
# hence defined).  Undef returns undef.
#
# the three flavors are slut(), easy() and saint().
#
# slut() always returns what you passed it.  a string like
# "myfilename; cd /; rm -rf *" is totally cool.  Needless to say,
# this should be used with extreme caution.  Trusted files (like
# configuration files) and really ugly data (i.e. the comment lines
# in database references).
#
# easy() allows most chars but excludes things known to be hazardous to
# the shell, i.e. non-displayable chars, '&', ';' '`', '|' '>', '<'
#
# saint() allows only alphanumerics and '_', '-', '.', ':' and '/'.
# It's intended mostly for (unix) file paths.