What's wrong with something like this:

sub parse_commands
{
    my ($filename) = @_;

    my $fh = IO::File->new($filename) || die ...;
    while (<$fh>)
    {
        # All commands being with a letter and must be the first chara
+cter
        next if /^[^A-Za-z]/;

        # A command is space-delimited and must be on one line.
        # We want to deal with the entire command in uppercase.
        my ($command, @line) = split /\s+/, uc;

        unless (exists $DispatchTable{$command})
        {
            warn "'$command' isn't a valid command.\n";
            next;
        }

        push @commands, [$command, \@line];
    }

    return \@commands;
}
[download]

All of a sudden, you have a parsed list of commands. Obviously, you'd do something a little more in-depth when dealing with WHILE/ENDWHILE, IF/ELSE/ENDIF, SUB/ENDSUB, and FOR/ENDFOR, but it's not that much more complicated. It can even be a one-pass compiler if you

• do all includes during the parsing phase (this requires reading the initial file first into an array for splicing)
• require all subroutines be defined before usage (if SUB is even allowed)
• require all labels to be on a line by themselves. (Something along the lines of /^([A-Z]\w*):$/)
• don't care if a label is defined until runtime.

If interested, I'll write one. Remember - the handler for each command is the one that cares about parameters and the like. This isn't a fully-featured language (though it is Turing-complete).