#!perl -w
use strict;

sub hello { print "Hello $_\n" for @_ };
sub bye { print "Goodbye $_\n" for @_ };

my $namespace = \%main::;

my ($command,@args) = @ARGV;
if (! exists $namespace->{$command}) {
    warn "Unknown command '$command'. Valid commands are:\n";
    for (sort keys %$namespace) {
        if (defined *{$namespace->{$_}}{CODE}) {
            warn "$_\n";
        };
    };
    die "$0 stopped.\n";
} else {
    my $code = $namespace->{$command};
    $code->(@args);
};

__END__

Q:\>perl -w tmp.pl x
Unknown command 'x'. Valid commands are:
bye
hello
tmp.pl stopped.

Q:\>perl -w tmp.pl hello Foo
Hello Foo

Q:\>perl -w tmp.pl bye Foo
Goodbye Foo

Q:\>