I'm not sure exactly what you mean by isachar, since that doesn't appear to be one of the ANSI C standard functions. However, taking Zaxo's code one step further:

sub isachar
{
    return pop =~ /^[\x00-\x7F]/;
}

# Or alternatively
sub isalnum
{
    return pop =~ /^\w/;
}

# As in:
$is = isachar('a');
[download]

Update:
crazyinsomiac pointed out "pop is not a variable", but it is a quick way of retrieving the first element of @_, a.k.a. $_[0].