This should work for simple cases (no newlines inside single quotes, no single quotes in comments):

#!/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
[download]

Updated.
To handle single quotes in comments, you might need to change the script to the following:

#!/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
[download]

To get the code instead of the comments, just invert the logic:

while (<DATA>) {
    chomp;
    my @code;
    my @items = split /'/;
    until (not @items or $items[0] =~ s/--.*//) {
        @items and push @code, shift @items for 1, 2;
    }
    print join("'", @code), @items ? (@code ? "'" : q() ) . "$items[0]
+"
                                   : q(), "\n";
}
[download]

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