And we were just discussing Dunning–Kruger the other day...

use strict;
use warnings;
use CGI::Pretty ();
use URI ();

my $cgi = CGI::Pretty->new();

print
    $cgi->header(),
    $cgi->start_html(),
    $cgi->h3("CAN I HAZ PURL?"),
    $cgi->h4("YUR DOIN IT RONG"),
    $cgi->blockquote( yours($cgi) ),
    $cgi->h4("YUR DOIN IT... PRETY GUD AKSHULY"),
    $cgi->blockquote( mine($cgi) ),
    $cgi->end_html();

exit 0;

sub yours {
    my $cgi = shift;
    $cgi->url(-relative=>1) . '?a=';
}

sub mine {
    my $cgi = shift;
    my $uri = URI->new( $cgi->url );
    ( my $path = $uri->path ) =~ s![^/]+\z!!;
    $uri->path( $path || "/" );
    $uri->query_form( a => undef );
    $uri;
}
[download]

CAN I HAZ PURL?

YUR DOIN IT RONG

pm-805175.cgi?a=

YUR DOIN IT... PRETY GUD AKSHULY

http://localhost/cgi/?a=

Jeez mother, I already corrected myself in Re^3: CGI Cookie Inconsistencies