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sub flag_status {
  my $v = $_[0];
...
 # return 16 if argument's IsUV flag is set
 # else return 0
}

my $d = 2.7;
my $flags_prior = flag_status($d);
...
my $flags_after = flag_status($d);
die "foo() altered its argument's numeric flags"
  if($flags_prior != $flags_after);