You may have better luck changing the code to:

my $ham = "spam\tspam\tspam\t\tyam\tclam";

my @jam = ($ham =~ (m/^[^\t]+\t[^\t]+\t([^\t]+)(\t\t)([^\t]+)\t[^\t]+$
+/));

print join("\n", '**', @jam, '**', '');
[download]

prints:

**
spam

yam
**
[download]