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open(my $fh, "<", "parameter.csv") or die "$!";
my $header = <$fh>;
...
    ${module}{$t}{indicator}{$n}{yscala} = $line[4];
}
close $fh;

my @cmds;
for my $number (sort keys %{${module}{$topic}{indicator}}){
...
for (my $i=0; $i<@cmds; $i++) {
        $slurpline.=$cmds[$i];
}

open my $P, "|-", "gnuplot" or die;

...
]; 

close $P;