The fundamental problem here is that lists and arrays are two different things. That's a FAQ

Lists are temporary data sequences which can't be changed!

But Arrays are variable(s) with @sigils and they are changed.

Shift can only operate on arrays, because they are shortened afterwards.

Once you've understood this...

> why can't I shift a split?

Because split returns a list!

Unless you assign it to an array it can't be shifted.

Addendum

Others have shown solutions with (lists)[slices] which work similar to @array[slices] . That's only possible because a slice doesn't change its object, unlike shift.

Further reading

• List's terminology and Parentheses in Perl
• Examples for "LIST", "list context", "list value", "list assigment", "list operator"

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