My thoughts were to just make MyCapture a subclass of Tie::Handle::Scalar that registers callbacks. Here's an example MyCapture.pm that is similar to your STDOUT::Capture module. You could go ahead and provide a wrapper module that hardcodes the desired callbacks as you do, or hardcode them directly in this subclass.

## -- MyCapture.pm -- ##
package MyCapture;
use Tie::Handle::Scalar;
our @ISA = qw/Tie::Handle::Scalar/;
my %callbacks;

sub import {
  my $self = shift;
  %callbacks = @_;
}

sub PRINT {
  my $self = shift;
  my $input = join '', @_;
  if(exists $callbacks{on_print}){
    $callbacks{on_print}->(\$input);
  }
  $self->SUPER::PRINT($input);
}

sub DESTROY {
  my $self = shift;
  if(exists $callbacks{on_finish}){
    $callbacks{on_finish}->($self->{data});
  }
  print ${$self->{data}};
}

1;
__END__

## -- test.pl -- ##
#!/usr/bin/perl -w
use strict;
use CGI qw(:standard);
use MyCapture on_finish => \&on_finish,
              on_print  => sub{${$_[0]} =~ s/\bcgi\b/CGI/g};

sub on_finish {
  my $input = shift;
  $$input =~
    s{(</head>)}
    {<link rel="stylesheet" type="text/css" href="/css/style.css" />\n
+$1}i;
}

tie *STDOUT, 'MyCapture';

print header;
print start_html(-title => 'Cool cgi program');
print h1('Welcome to my cool cgi!');
print p('This is just a test page.');
print end_html;

untie *STDOUT;

__END__
[download]