sub foo
{
    my $val;
    if(  @_  ) {
        $val= shift;
    } else {
        undef $val;
    }
    return $val;
}
foo( "x" x 10_000 );
# 10,000 bytes of memory still tied up
foo()
# those 10,000 bytes now free to be reused
foo( \( "x" x 10_000 ) );
# the few bytes needed to store a reference still tied up,
# but the 10,000 bytes for the string value itself are
# already free to be reused.