How do I extract selected columns from a string?

Use substr() or unpack(), both documented in perlfunc. If you prefer thinking in terms of columns instead of widths, you can use this kind of thing:

    # determine the unpack format needed to split Linux ps output
    # arguments are cut columns
    my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);

    sub cut2fmt { 
	my(@positions) = @_;
	my $template  = '';
	my $lastpos   = 1;
	for my $place (@positions) {
	    $template .= "A" . ($place - $lastpos) . " "; 
	    $lastpos   = $place;
	}
	$template .= "A*";
	return $template;
    }