open my $dfout, "df -k|" or die "couldn't start df -k: $!";
open DISKSPACE as you do
while (<$dfout>) {
  ...
}
close $dfout or die "df failed: $! $?";
[download]

This is the current code and the error returned when ran.

format DISKSPACE =
($Filesystem,$kbytes, $used, $avail, $capacity, $Mounted_on) = split (
+/ /);

======================
|@<<<<<<<<<<<<<<<<<<<|
$Filesystem
|@<<<<<<<<<<<<<<<<<<<|
$kbytes
|@<<<<<<<<<<<<<<<<<<<|
$used
|@<<<<<<<<<<<<<<<<<<<|
$avail
|@<<<<<<<<<<<<<<<<<<<|
$capacity
|@<<<<<<<<<<<<<<<<<<<|
$Mounted_on
======================
.


sub df_total {

#    if ($answer eq "F") {
#    write ($answer);

open (DISKSPACE, "< spaceavail.txt") || die "Error opening file spacea
+vail.txt";

open (DISKSPACE, "> spaceavail.txt") || die "Error opening file spacea
+vail.txt";
while (<DISKSPACE>) {
  chop;
  ($Filesystem, $kbytes, $used, $avail, $capacity, $Mounted_on) = spli
+t (/ /);
  write (DISKSPACE);
close (DISKSPACE)
}
}

print "\n  My First Perl Menu\n";
print "A.  Current Date & Time\n";
print "B.  Users currently logged in\n";
print "C.  Name of working directory\n";
print "D.  Contents of the working directory\n";
print "E.  Names & Login names of user currently logged on to aries\n"
+;
print "F.  Aries Disk Space Utilization\n";

print "Enter A, B, C, D, E, F: \n";

$answer = <STDIN>;
chomp ($answer);

if ($answer eq "A") {
      system "date \n";
}
elsif
    ($answer eq "B") {
      system "who \n";
}
elsif
   ($answer eq "C") {
     system "pwd \n";

}

elsif
   ($answer eq "D") {
     system "ls -Fla \n";
}

elsif
   ($answer eq "E") {
     system "finger \n";
}

elsif
   ($answer eq "F") {
     system "df -k \n";
     &df_total
}

else {
   print "The selection you entered: $answer, is incorrect. Please ent
+er A, B, C
 or D \n";
}




$ ./testmenu.pl

  My First Perl Menu
A.  Current Date & Time
B.  Users currently logged in
C.  Name of working directory
D.  Contents of the working directory
E.  Names & Login names of user currently logged on to aries
F.  Aries Disk Space Utilization
Enter A, B, C, D, E, F:
F
Filesystem            kbytes    used   avail capacity  Mounted on
/dev/dsk/c1t0d0s0    2016343 1392038  563815    72%    /
/proc                      0       0       0     0%    /proc
mnttab                     0       0       0     0%    /etc/mnttab
fd                         0       0       0     0%    /dev/fd
swap                 3888816      40 3888776     1%    /var/run
swap                 3888848      72 3888776     1%    /tmp
/dev/dsk/c1t0d0s7    32442879  523718 31594733     2%    /export/home
Filehandle DISKSPACE opened only for output at ./testmenu.pl line 46.
[download]

You are trying to use the same filehandle DISKSPACE for reading the output of df and for writing your formatted result. Get rid of the system "df -k \n"; in the main code, change your first open in df_total to:

open DISKSPACEIN, "df -k|" or die "couldn't start df -k: $!";
[download]

(The | means to execute the given command and make it's output available via the filehandle. See perlipc.)

Then change your while (<DISKSPACE>) to while (<DISKSPACEIN>).

It's also an especially good idea when using pipes (the "df -k|" argument to open makes what's called a pipe) to explicitly close the filehandle and check the result, so after the while loop, put:

close DISKSPACEIN or die "got df error: $! $?";
[download]

See perlipc for more about this.