You probably can get away with tieing the filehandle with Tie::Handle. Here's an example:

package html;
require Tie::Handle;
@ISA = qw/Tie::Handle/;

use HTML::Entities;

sub TIEHANDLE {
    return bless({}, shift);
}

sub PRINT {
    untie *STDOUT;
    print encode_entities(join "", @_);
    tie *STDOUT, "html";
}

package main;

tie *STDOUT, "html";
print "foo & bar\n";
[download]

It might need some work to plug cleanly in whatever code you need to write.

--
Leviathan.