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

void modSV( SV *ref ) {
    SV *sv = SvRV( ref );
    unsigned __int64 uv = 9223372036854775809ull;

    SvUV_set( sv, uv );
    SvIOK_only_UV( sv );
    return;
}

END_C

use Devel::Peek;

my $sv = 123;
Dump $sv;

print modSV( \$sv );
Dump $sv;

__END__
SV = IV(0x2c7490) at 0x2c7498
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 123

SV = IV(0x2c7490) at 0x2c7498
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = -9223372036854775807