By changing your rules a bit (since you aren't 100% on all of them are they under your control?) can't you just use string eval to let the perl interpreter apply its own rules?

my $str = "123e4";
print asNum( $str );
sub asNum {
  my $str = shift;
  my $foo;
  eval "\$foo = $str"; # backslash for string not reference
  if( $@ ) {
    warn( "oops : $@" );
  }
  return $str;
}
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Ugly, but probably 'rock-solid', no esoteric modules and should work with any perl.