causes a die or a warn to render as nice HTML in a web-browser for debugging purposes

Check this out:

#!/usr/bin/perl -w

use strict;
use CGI qw/ :all /;
$SIG{__DIE__} = \&htmldie;
open FIN,"< some_stupid_nonexistant_file.or.other" or die $!;

my $line=<FIN>;

exit(0);

sub htmldie {
        my @args=@_;
        print header,start_html,pre(join("\n",@args)),end_html;
        exit(0);
}
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I found this by doing a quick check of perldoc -f die which shows this pattern.