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

while ( (print "Enter the string(or Quit): "), 
        (my $line = <STDIN>) !~ /^\s*q(uit)?\s*$/i 
      )
{
   next if ($line =~ m/^\s*$/); #re-prompt if blank line
   $line =~ s/^\s*//;           #delete leading  spaces
   $line =~ s/\s$//;            #delete trailing spaces
   
   print "Enter the Sub-string size: ";
   my $size = (<STDIN>);

   unless(      ($size =~ m/^\s*\d\s*$/) #only single digit
            &&  (length($line) >= $size) #only valid context
         )
   {
      print "Invalid size, must be a single positive digit\n";
      print "   and less than or equal to: ",length($line),"\n";
      next;
   }
   
   print "output: ",substr($line,0,$size),"\n";
}

__END__
Example Output:

C:\TEMP>perlcommand.pl
Enter the string(or Quit): asdf
Enter the Sub-string size: 8
Invalid size, must be a single positive digit
   and less than or equal to: 4
Enter the string(or Quit): asdf
Enter the Sub-string size: -23
Invalid size, must be a single positive digit
   and less than or equal to: 4
Enter the string(or Quit): asdf
Enter the Sub-string size: 2
output: as
Enter the string(or Quit): assf
Enter the Sub-string size: 4
output: assf
Enter the string(or Quit): q