$self->content(sub{s/foo/bar/g});

##</code><code>##

sub content {
    my $self = shift;
    if (@_) {
        my $set = shift;
        if (isa($set,"CODE")) {
            # fixed :-)
            &$set for $self->{'content'}; # Code to modify content (as $_) gets executed
        }
        else {
            $self->{'content'} = $set;   # Direct set
        }
    }
    $self->{'content'};
}

##</code><code>##

$self->content =~ s/foo/bar/g;
# instead of
# $self->content(map { s/foo/bar/g } map $_, $self->content);