#!/usr/bin/perl
use warnings;
use strict;

while (<DATA>) {
    my @items = split /'/;
    until (not @items or $items[0] =~ s/.*?--/--/) {
        shift @items for 1, 2;  # Remove the quoted part, too.
    }
    print join "'", @items;
}

__DATA__
        select 'text' from foo		--This is a comment
        select '--Not a valid comment' from foo		--But this is
        select q from z -- as is this
        select '--This is not a valid comment' from foo
        select '--Not this' + '--either' from foo
        select 'qaws' + make from "a" -- comment with 'a' quote
        select 'a' from 'b' with 'c' -- comment with 'a --' comment