$infile = "outfile.txt";
open (INFILE, "$infile");

$out_b = "outfile_b.txt";
open (OUT_B, "+>>$out_b");

while(<INFILE>){   
    print "here\n";
    
    if ($_ =~ /^(.{1,50})\t{0,50}(.{0,50})\t{0,1}(.{0,50})\t{0,1}(.{0,50})\t{0,1}(.{0,50})\t{0,1}(.{0,50})/){
	
	print "here_B\n";

	$gene_id = $1;
	if ($2 = /.{1,50}/){ (@alternative_ids, $2)};
	if ($3 = /.{1,50}/){ (@alternative_ids, $3)};
	if ($4 = /.{1,50}/){ (@alternative_ids, $4)};
	if ($5 = /.{1,50}/){ (@alternative_ids, $5)};
	if ($6 = /.{1,50}/){ (@alternative_ids, $6)};
	if ($7 = /.{1,50}/){ (@alternative_ids, $7)};
	if ($8 = /.{1,50}/){ (@alternative_ids, $8)};
       	if ($9 = /.{1,50}/){ (@alternative_ids, $9)};
	print @alternative_id; 
	foreach (@alternative_ids){
	    
	    $alternative_id = $_;
	    print OUT_B "$gene_id\t$alternative_id\n"; 
	    
	}
	undef @alternative_ids;
    }
}