The right way to decide if some value is between say 10 and 15 is to use 2 compares and "AND" the result together. The value must be >=10 AND <=15. Below I used the && operator instead of "and" which would have worked also. There is a difference in precedence between these 2 operators either one will work with the statement as I have written it.

In order to get your code to print "Yes", I set $frequency = 10. This winds up meaning: "if ($frequency == 10 ...some not understandable stuff, hence the error message)".

The "==" operator means exactly "equal to".

I recommend always enabling warnings, either by the "shebang line" #!/usr/bin/perl -w or the statement "use warnings;". "use strict" is also an extremely good idea.

You would have seen immediately that Perl didn't like your "if" statement.

#!/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!
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