The solution depends a bit on your problem. If you ever plan to treat the string as a hex number in Perl then my first take on the problem would be to use Perl's own hex() function to see if Perl thinks it's a hex number or not. This routine will return false for any complaints Perl might have, like integer overflow or non-portable value.

sub is_hex {
  my ($s) = @_;

  local $SIG{__WARN__} = sub { die };
  return eval { scalar hex $s; 1 };
}
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If you don't care about if Perl thinks it's portable and/or too large to be an integer then you can make it not warn about that through the warnings pragma (no warnings 'appropriate category';, see perllexwarn) or by handling the warnings in the warn hook:

local $SIG{__WARN__} = sub {
  /non-portable/i or
  /overflow/i or
  die for @_;
};
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Hope I've helped,
ihb