Okay. I found a way to make it go away. Initialising out silences the warning:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'Endian', CLEAN_AFTER_BUILD =>0;

SV *hexDump( UV in ) {
    int i;
    char *p = (char*)&in;
    SV *out = newSVpvn( "?", 1 );

    for( i=0; i<8; ++i ) {
        sv_catpvf( out, "%02x", p[i] );
    }
    return out;
}

END_C

print hexDump( 72623859790382856 );

__END__
C:\test>endian
?0807060504030201
[download]

Which I guess means that the doc for newSVpvn() that reads:"If the s argument is NULL the new SV will be undefined. means something different to what I took it to mean.

This also works:

SV *hexDump( UV in ) {
    int i;
    char *p = (char*)∈
    SV *out = newSVpvn( "", 0 );

    for( i=0; i<8; ++i ) {
        sv_catpvf( out, "%02x", p[i] );
    }
    return out;
}
[download]