sub foo {
    my ($file, $selector_name) = @_;
    
    # Create CSS object
    use CSS;
    my $css = CSS->new;
    
    # Read CSS from file passed as first arg to foo
    $css->read_file($file);
    
    # Get CSS::Style object associated with the name of the
    # selector passed as the second arg to foo
    my $style = $css->get_style_by_selector($selector_name);
    
    # Get selectors (serialized)
    my $selectors = $style->selectors;
    
    # ...
}

# Example call (as I have foo, it returns nothing, you
#               can fill that in)
my $ret = foo('styles.css', 'body');