my %supermatrix; 
my $list = "LIST.txt";
open my $DATA, '<', $list or die "unable to open my list $list $!\n"; 
print "\nWORKING ON $list\n";
print `head $list`;
while (my $line = <$DATA>)  { 
     chomp $line; 
     
     my $substrate = 1 if $line =~ /substrate.*(\d+)$/;
     print "SUBSTRATE ===$substrate";  
     $line = <DATA>; # fetch next line
     print "PRODUCT====$line"; 
     my @products; 
     (undef, @products) = split / /, $line; 
     foreach my $prod (@products) { 
          $supermatrix{$substrate}{$prod} = 1;
          
          print "combination $substrate $prod exists ! \n" if exists $supermatrix{$substrate}{$prod};
     } 
}