use strict;
use warnings;

$| = 1; # auto flush stdout

sub more { print "more()\n" }
sub fallover { print "fallover()\n" }
sub leave { print "Bye!\n"; exit; }

my %cmdhash = (
  fun => \&more,
  laugh => \&fallover,
  exit => \&leave,
);

  

print "Welcome to johns fun house\n";
while (1) {
  print " : ";
  my $command=<>;
  chomp $command;
  if (my $func = $cmdhash{$command}) {
    $func->();
  } else {
    print "I'm afraid I don't know how to \"$command\"\n";
  }

}
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$| = 1; # auto flush stdout

Why?

Because that's how you ensure the " : " prompt will be written out before the readline.
--
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When I ran this without that, I didn't see the prompt or any of the output. I was running ActivePerl 5.8.8 in an rxvt window. If I run cygwin perl, I don't need the $|=1, or if I run either perl in a DOS box, I don't need it. Dunno why.