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

my $frequency = 10;
if ($frequency == 10 .. 15)  #this is line 5
{print "Yes freq 10 is ok!(or maybe not quite right!)"}

print "\n";
$frequency = 12;
if ( (10 <= $frequency) && ($frequency <=15 ) ) 
{
   print "Yes $frequency is ok!\n";
}

__END__
prints:
Use of uninitialized value $frequency in range (or flop) 
      at C:\TEMP\compare.pl line 5.
Yes freq 10 is ok!(or maybe not quite right!)
Yes 12 is ok!