I did something similar for creating absolute paths for use in redirects. Here's the code:

sub absolute_url ($$) {
    my ($base,$url) = @_;
    $base =~ s{/[^/]*$}{/};                 # remove file from base
    $url = $base.$url;                      # append base and url
    1 while ( $url =~ s{/[^/]*/\.\./}{/} ); # remove ..
    return $url;                            # all done!
}
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