Just shooting from the hip, I think you want to replace

my $cmd = "perl ../../../WP4_utility_analysis/WP4_utility.pl $slot_fil
+e $SLOT_OUTFILE";
[download]

with

my $cmd = "perl ../../../WP4_utility_analysis/WP4_utility.pl $slot_fil
+e >$slot_outfile"
[download]

Then you need to have your second script print to STDOUT rather than $OUTFILE.