> "If more than one variable is listed, the list must be placed in parentheses"

means:

If VARLIST is

$x,$y,$z

you must write

my ($x,$y,$z)

The alternative

my $x,$y,$z

would only declare $x because of precedence, like

(my $x),$y,$z

Hence an alternative to what we want is

my $x,my $y,my $z

But since we need a list assignment on the LHS, we still need to put it all into parens

(my $x,my $y,my $z) = split ...

because this

my $x,my $y,my $z = split ...

would only assign to $z because of precedence°, like

(my $x),(my $y),(my $z = split ...)

HTH! :)

Errata

s/declare/assign to/ choroba++

UPDATES

°) Even worse, it would only be a scalar assignment to $z, i.e. the number of possible splits.