By placing the comma in a negated character class you lose any character preceding the tab, apart from a comma, in the resultant array because it becomes part of the split term. Placing the comma in a negative zero-width look-behind, as gmargo does here, the characters are retained.

$ perl -le '
> $txt = qq{abc\tdef,\tghi\tjkl\tmno};
> print for split m{[^,]\t}, $txt;
> print q{-} x 20;
> print for split m{(?<!,)\t}, $txt;'
ab
def,    gh
jk
mno
--------------------
abc
def,    ghi
jkl
mno
$
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I hope this is of interest.