You could write that as

sub foo {
    my $var = @_ ? \$_[0] : \$_;
    modify($$var);
}

foo for @list;
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Beware of false laziness however. You should not use this kind of interface lightheartedly. Are you sure it is a better idea than just wrapping the inside of &foo with a for loop?

sub foo {
    for(@_) {
        modify($_);
    }
}

foo @list;
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Makeshifts last the longest.