sub bot_command {
    my $command = shift(@_);

    join_channel($command) if ($command =~ /join/);
    part_channel($command) if ($command =~ /part/);
    msg_user($command)     if ($command =~ /msg/);
    ....
    default($command);
}

##</code><code>##

sub bot_command {
    local $_ = shift;

    /join/ ? &join_channel :
    /part/ ? &part_channel :
    /msg/  ? &msg_user     :
             &default
}

##</code><code>##

  sub join_channel { ... }
  sub part_channel { ... }
  sub msg_user { ... }

  my %dispatch = (
    join => \&join_channel,
    part => \&part_channel,
    msg  => \&msg_user,
  );

  sub bot_command {
    my $cmd = shift;
    $dispatch{$cmd}
       ? &{$dispatch{$cmd}}
       : &default
  }