$ perl -le'
sub head($)
{
    my ($x, $xs) = split( " ", ${$_[0]}, 2);
    ${$_[0]} = $xs;
    return $x;
}
my $msg = "new item x";
my $cmd = head \ $msg;
print for $cmd, $msg;
'
new
item x

$ perl -le'
sub head   
{
    my ($x, $xs) = split( " ", $_[0], 2);
    $_[0] = $xs;
    return $x;
}
my $msg = "new item x";
my $cmd = head $msg;
print for $cmd, $msg;
'
new
item x