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: $!";
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(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: $! $?";
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See perlipc for more about this.