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

while (<DATA>) {
    my $last = (split /'/)[-1];
    print $1 if $last =~ /(--.*)/;
}

__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