Other ideas to try.

Just push the pairs onto the stack and let the user assign that to a single level array:

my @diffs = diff( ... );
while( @diffs ) {
    my( $x, $y ) = ( pop @diffs, pop @diffs );
    ## use em.
}
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Build a single level C array and return a SvPV that points to the head of that array:

U32 len = sizeof( U32 ) * some over estimate;
U32 *diffs =  malloc( len );
U32 i = 0;
SV *packed;

SvPV_Set( packed, diffs ); SvLEN_set( len )

for( ) {
    for( ) {
        diffs[ i++ ] = x;
        diffs[ i++ ] = y;
    {
}

SvCur_set( packed, ( i-1 ) * sizeof( U32 ) );

return packed; // sv_2mortal?
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then use unpack to access the numbers:

my $diffs = diff( ... );

while( length( $diffs ) ) {
    my( $x, $y ) = unpack 'VV', substr( $diffs, 0, 8, '' );
    ## use em.
}
[download]