shift requires an array (@NAME, @BLOCK, EXPR->@* and similar).

---

You want a list slice (( LIST )[ INDEXES ]).

Specifically,

( split( /,/, $_, 2 ) )[ 0 ]
[download]

---

If you are assigning the result to a scalar, you can also use

my ( $first ) = split( /,/, $_, 2 );
[download]

---

Using 2 for split's third operand isn't required; it's an optimization. There's no reason to continue splitting after the first split.