my @row = split("\t", chomp(<IN>));

chomp() returns the number of characters removed, not the chomped values. So even if you were to sidestep your immediate problem by doing

my @row = split("\t", chomp(my $line = <IN>));
[download]

You still wouldn't get what you're expecting.

chomp() is a special beast. It modifies its arguments. You can't modify a file handle in the same way.