validate chars for a string

[perl]

Since, everyone is so lively today, I'm going to push my luck today with
this list. I need a little help here with validating a string to have only
characters that I want, A-z and _ (underscore).

So, this is what I'm stuck on:

#should fail because of space
$username="bob by";

if(&isValidChars($username))
 ...

#is there a better way but it isn't working anyway.
sub isValidChars
{ my $retVal;

  @chars=split(//,$_[0]);

  for(@chars)
  {
    if($_ =~ /A-z/) { print "good [" . $_ . "]\n"; $retVal=1; }
    else { print "bad [" . $_ . "]\n"; $retVal=0; break; }
  }
  return $retval;
}


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