#!/usr/bin/perl -w
use strict;

#basic "command loop"

while ( (print "Enter a command: "), 
        (my $line = <STDIN>) !~ /^\s*q(uit)?\s*$/i  
      )
{
 #### Validate input and if ok, call a sub at the
 #### of validation
 
   next unless $line =~ m/\S/;
   #chomp ($line);  #actually optional here (\n counts as $)
   
   if ($line =~ m/^\s*help\s*$|^\s*h\s*$/i)
   {
      print "there is no help for the helpless!!!\n";
      next;
   }

   if ($line =~ m/^\s*[+-]{0,1}\s*\d+\s*$/)
   {
      print "wow, a number! try again!\n";
      next;
   }
   
   if ($line !~ m/^add$|^del$/i)
   {
      print "invalid_command!!\n"; 
      next;
   }
   
   add() if $line =~ m/add/;
   del() if $line =~ m/del/;
}
sub add
{ print "some kind of add happend\n";}
sub del
{ print "some kind of del happened\n";}

__END__
Some example interaction:

C:\TEMP>perl stdcommandloop.pl
Enter a command: 34
wow, a number! try again!
Enter a command: -45
wow, a number! try again!
Enter a command: +-56
invalid_command!!
Enter a command:
Enter a command:
Enter a command: help
there is no help for the helpless!!!
Enter a command:
Enter a command: del
some kind of del happened
Enter a command: add
some kind of add happend
Enter a command: quit
> (OS prompt is back, end of program)