I had a quick look at the source code and it seems that if you were to put some of the things which the filter is replacing (eg the sub declerations) inside a quoted string they would still be replaced even though they clearly shouldn't.

No! I use Filter::Simple, and I parse subs and classes only in the 2nd part of the filter, where strings are in a place holder:

FILTER_ONLY( all => \&filter_html_blocks , code => \&CLASS_HPLOO , all
+ => \&dump_code ) ;
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Soo, I take care about quoted strings! ;-P

Here's a valid code:

use Class::HPLOO ;#qw(dump) ;

  print "class null { sub n { 1 } }\n" ;
  
class Foo {

  sub test {
    print "test>>@_\n" ;
    print " sub fake { 2 } \n" ;
  }

}
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Generated code:

  print "class null { sub n { 1 } }\n" ;
  
{ package Foo ;

  use strict qw(vars) ;
 
  sub new { 
    my $class = shift ;
    my $this = bless({} , $class) ;
    my $ret_this = $this->Foo(@_) if defined &Foo ;
    $this = $ret_this if ( UNIVERSAL::isa($ret_this,$class) ) ;
    $this = undef if ( $ret_this eq '0' ) ;
    return $this ;
  }


  sub test { 
    my $this = shift ;
    
    print "test>>@_\n" ;
    print " sub fake { 2 } \n" ;
  }


}
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