It is ambiguous. You might mean shift(), or you might mean @shift. The construct @{ shift } could mean either thing. See, for example:

use strict;
use warnings;

$, = "\t";

my @foo = qw/this that/;

print @{ foo }, "\n";

my @shift = qw/those these/;

print @{ shift }, "\n";

mysub( [qw/them theirs/] );

sub mysub {
    print @{ shift }, "\n";
    print @{ shift() }, "\n";
}
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