use URI, and in particular, use URI::URL; The documentation is pretty self explanatory.

update: somebody asked, so here is an example

perl -MURI::URL -e"$URI::ABS_REMOTE_LEADING_DOTS=1; die URI::URL->new(
+'../../foo.html','http://foo.org/a/b/c/')->abs"
http://foo.org/a/foo.html

#or if that doesn't work for you
# the path method will do nicely
use URI::URL;
$URI::ABS_REMOTE_LEADING_DOTS=1;
my $ur = new URI::URL('/foo/../bar/baz.html','http://foo.org/');
my @path = ();

for my $seg($ur->path_components()) {
    next unless $seg;

    if($seg eq '..') {
        pop @path;
    } else {
        push @path, $seg;
    }
}
print join '/',@path;
[download]

update:a slightly improved (interesting) example #!/usr/bin/perl -w #

use strict;
use URI::URL;
$URI::ABS_REMOTE_LEADING_DOTS=1;
my $ur = new URI::URL('/foo/../bar/baz.html','http://foo.org/');

my $path = "";
my @path;
for my $seg($ur->path_segments()) {

    unless($seg) { # it began with a /
        $path .= "/";
        next;
    }

    if($seg eq '..') {
        pop @path;
    } else {
        push @path, $seg;
    }
}

$\="\n";
$path .= join '/',@path;
print $path;

$ur = new URI::URL($path,'http://foo.org/');

print $ur->abs();
print $ur->canonical();
print 'this is interesting';
print URLo($path.'?ab=cd;ef=gh;#foy','http://foy.org');
# note no trailing slash on  foy.org


sub URLo{
    use CGI qw/ a b big /;
    my $url = URI::URL->new(shift);
    my $www = shift;
    my $ret = a({-href => $www}, $www);
    my $dir = "";

    for $dir ($url->path_segments()) {
        next unless($dir); # it happens, like on /foo

        $www .= '/';
        $ret .= big( b('/') );

        $www .= $dir;
        $ret .= a({-href => $www}, $dir);
    }

    if(($dir = $url->query()) and $dir) {
        $www .= '?'.$dir;
        $ret .= big(b('?'));
        $ret .= a({-href => $www}, $dir);
    }

    if(($dir = $url->frag()) and $dir) {
        $www .= '#'.$dir;
        $ret .= big(b('#'));
        $ret .= a({-href => $www}, $dir);
    }

    return $ret;
}

__END__
/bar/baz.html
http://foo.org/bar/baz.html
/bar/baz.html
this is interesting
[download]

http://foy.org/bar/baz.html?ab=cd;ef=gh;#foy

 

______crazyinsomniac_____________________________ Of all the things I've lost, I miss my mind the most. perl -e "$q=$_;map({chr unpack qq;H*;,$_}split(q;;,q*H*));print;$q/$q;"