package something;

sub new{ bless {}, __PACKAGE__ }
sub doit {
    my ($self, $subref, $key) = @_;
    local $_ = $self->{ $key };
    $subref->();
    $self->{ $key } = $_;
}

package main;

my $o = something->new;

$o->doit(sub { $_ += 1 }, 'fred');

print "'$o->{fred}'\n"; # Outputs 1