For what it's worth, I took a look at your code and I've tried to give you a few helpful hints on another way to do it. I've placed my comments inline.

#!/usr/bin/perl
#
# Sorry for the simplicity of this script, but as a first submission..
+.
#
# Emails/Pages me when a filesystem > 90% full
#

# always use strict
use strict;

# make a variable to easily change some behavior
my($warn_percentage) = 90;

# predeclare our variables (not necessary, but probably a good idea)
my(%df);   # df hash that holds all our information
my($host); # hostname

# Rather than 3 backticks for df, just do it once and parse it with pe
+rl!
# Also, make $fs into an array instead of string then split to array
# Finally, we don't really need $full, since that's just grep /100/,@f
+s
#$df=`df -lk 2>/dev/null`;
#$fs=`df -lk 2>/dev/null | grep -v used | cut -d'%' -f2 | grep -v deni
+ed`;
#$full=`df -lk 2>/dev/null | grep -v used | cut -d'%' -f1`;
foreach (`df -lk 2>/dev/null`) {
    if (/(\b\d+)%\s+(\S+)$/) { $df{$2} = $1; }
}

# removed some string manipulation that isn't needed any longer
#$full =~ s/.*(..\d)$/\1/mg;
#$fs=~s/\s+/ /g;
#$full=~s/\s+/ /g;
#@fs=split(" ",$fs);
#@full=split(" ",$full);

# We can combine this into one statement
#$host=`hostname`;
#chomp($host);
chomp($host = `hostname`);

# this loop changes somewhat
#$fs=@fs;
#chomp($date); # where is $date coming from?

# note that the mailx stuff isn't tested, as I don't have mailx on my 
+system
foreach (keys %df) {
    if ($df{$_} > $warn_percentage) {
    print "Host: $host Filesystem: $_ is $df{$_}% full!\n";
    system("mailx -s \"$host:$_ is $df{$_}%\" youremail\@host.com << /
+dev/null");
    }
}

#while ($fs) {
#    $fs=shift(@fs);
#    $full=shift(@full);
#    chomp($fs);
#    chomp($full);
#    if ($full > 90) {
#    print "Host: $host Filesystem: $fs is ${full}% full!\n";
#    @chk=grep(/$fs:$full/,@log); # not sure what all this log stuff i
+s...
#    undef(@log);
#    $chk=@chk;
#    close(LOG);
#    }
#    if ($chk == 0) {
#    `mailx -s "$host:$fs is ${full}%" youremail\@host.com <<EOF
#$df
#EOF
#`;
#    `mailx -s "$host:$fs is ${full}%" mypager\@pagerhost.com <<EOF
#EOF
#`;
#    }
#}
[download]

Hope that helps!