Always use strict; and use warnings; if you are developing, helps a lot:

This a little bit "strictified" version of your code

#!/bin/env perl
#Look in daily syslog file and grab the
#user, group, connection duration, bytes xmt and bytes rcv
#Stick this data into a daily report in .csv format
#
use warnings;
use strict;

#TIME & DATE STUFF
my $stime = localtime;
my @time1 = split(' ', $stime);
my $mon = $time1[1];
my $day = $time1[2];
my $year = $time1[4];

my $filename = "dailyreport-$mon-$day-$year.csv";

#Open the log file for reading

open (LOG, "</var/log/ciscon.log") || die "Couldn't open log file $!";
open (TMP, ">>tmpfile.txt") || die "Couldn't open tmp file $!";
open (DREPORT, ">>/root/reports/$filename") || die "Couldn't open csv 
+$!";
print DREPORT "DAILY LOG FOR $mon $day $year\n";
print DREPORT "\n";
print DREPORT "USERNAME,GROUP,DURATION,BYTES XMT,BYTES RCV,\n";
print DREPORT "\n";

#Pump data into array and parse for usefull data
#Usefull data goes into tmpfile

while( <LOG>)
        {
        my @tmp = grep {/disconnected/} $_;
        print TMP "@tmp";
        }

#Close stuff we dont need

close TMP;
close LOG;

#Open temp file and stuff into array by spaces print data we
#want into the daily csv file

open (TMP1, "<tmpfile.txt")  || die "Can't read from tmpfile $!";
while( <TMP1> )
        {
        chomp $_;
        my @tmpa = split(' ', $_);
        print DREPORT "$tmpa[15],$tmpa[17],$tmpa[20],$tmp[23],$tmp[26]
+\n";
        }
close TMP1;
unlink ("tmpfile.txt");
[download]

yields:

[1823]tom@margo perl $ perl -c test3.pl
Global symbol "@tmp" requires explicit package name at test3.pl line 5
+0.
Global symbol "@tmp" requires explicit package name at test3.pl line 5
+0.
test3.pl had compilation errors.
[download]

I guess you mean tmpa in both cases, but I may be wrong...