The code I am trying to call back is currently just

sub recorder {
    print "@_";
}
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However, I just checked the address I am passing in from Perl, and the address the SetCallback code receives, and they are different? Which explains why it doesn't work, and probably the error message, but not why/how/where the address is being changed. The calling code looks like this:

        printf "Address passed from Perl to C: %x\n", \&recorder;
        setHooks( \&recorder, \&player ) and die "$^E";
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and setHooks() like this

SV *g_rec = 0, *g_play = 0;

int setHooks( SV* rec, SV* play ) {
    HANDLE hHook = GetModuleHandle( "hook" );

    printf( "SH rec:%x\n", rec );
    g_rec =  rec;
    SvREFCNT_inc( g_rec );
    printf( "SH g_rec:%x\n", g_rec );

//    SvREFCNT_inc( g_play = play );
    ...
}
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And the output from those print and printf statements on either side of the call is:

Address passed from Perl to C: 1950428
SH rec:1d2ef20
SH g_rec:1d2ef20
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So, at least I now know where it's going wrong, if not why. Thanks for your help.