Something like the following script will work, once you flesh it in.

#!/usr/bin/perl -Tw

use strict;
use CGI qw(:standard);

my $query = new CGI;

print header();

if ( defined $query->param('left') ) {
    # emit the left frame
    print start_html();
    print h1("Left");
    print end_html();
}
elsif ( defined $query->param('right') ) {
    # emit the right frame
    print start_html();
    print h1("Right");
    print end_html();
}
else {
    my $script = $ENV{'SCRIPT_NAME'};
    print frameset(
            {-cols => "25%,75%"},
            frame({-name => "left",  -src => "$script?left=1"}),
            frame({-name => "right", -src => "$script?right=1"})
          );
}
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