Tweak Metaphone.xs by adding #undef free as shown below, and the problem will probably go away:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "metaphone.h"

MODULE = Text::Metaphone        PACKAGE = Text::Metaphone

PROTOTYPES: ENABLE

SV *
Metaphone(word, ...)
        char* word
        PROTOTYPE: $;$
        PREINIT:
            int max_length = 0;
        INIT:
            char* phoned_word;
        CODE:
            if( items > 1 ) {
                max_length = SvIV(ST(1));
            }
            metaphone(word, max_length, &phoned_word);
            RETVAL = newSVpv(phoned_word, 0);
#undef free            
            free(phoned_word);

        OUTPUT:
            RETVAL
[download]

If this works for you, contact the module author and suggest the change to him.

Rational: The memory being freed is allocated using the C-runtime's malloc(), but the XS environment redefines free() to use it's idea of what that function should be, and it is in that redirected function things go awry. (Which is totally bogus because they haveSafefree() for memory allocated by the Perl allocator.)

By using #undef free before calling it, you get back the CRT free, and the error message and subsequent traps go away (in those cases I've tried).