sub abs_url {
    my ( $relative, $base ) = @_;

    return $relative if $relative =~ m{ \A http:// }ix;

    my ( $host, $hostrelative_abs ) = $base =~ m{
        \A
        http:// # skip scheme
        ([^/]*) # capture hostname
        /*      # skip front slashes
        (.*?)   # capture everything that follows, but
        [^/]*   # leave out the optional final non-directory component
        \z
    }ix;

    $hostrelative_abs = '' if $relative =~ m!^/!;

    my $abs_url = join '/', $host, $hostrelative_abs, $relative;

    # replace '//' or '/./' with '/'
    1 while $abs_url =~ s{ / \.? (?=/|\z) }{}x;

    # remove '/foo/..' (but be careful to skip '/../..')
    1 while $abs_url =~ s{ / (?!\.\.) [^/]+ / \.\. (?=/|\z) }{}x;

    return "http://$abs_url";
}

use Test::More;
my @test = qw(
 /foo/bar.gif      http://www.example.com/baz/quux/    http://www.exam
+ple.com/foo/bar.gif
 foo/bar.gif       http://www.example.com/baz/quux/    http://www.exam
+ple.com/baz/quux/foo/bar.gif
 ../../bar.gif     http://www.example.com/baz/quux/    http://www.exam
+ple.com/bar.gif
 foo/bar.gif       http://www.example.com/baz/quux     http://www.exam
+ple.com/baz/foo/bar.gif
 ../foo/bar.gif    http://www.example.com/baz/quux     http://www.exam
+ple.com/foo/bar.gif
 ../../foo/bar.gif http://www.example.com/baz/quux/qux http://www.exam
+ple.com/foo/bar.gif
);
plan tests => @test / 3;
do { is abs_url( $test[0], $test[1] ), $test[2]; splice @test, 0, 3 } 
+while @test;