Have I got my rules for scoping wrong, or is that code useless?

my $ro_string = "    I'm surrounded by spaces!    ";
for ($ro_string) { # alias $_ to $ro_string for this scope
    s/^\s*//; # strip leading spaces from $_
    s/\s*$//; # strip trailing spaces from $_
} # lose $_ when the scope ends
print; # nothing!
[download]

At least, that's what I'm getting on Perl 5.8.9.

UPDATE: True but irrelevant, as pointed out by Your Mother and ikegami.