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!
[download]

Nicely done. Thank you all for advice and insight.

I think that i'll opt for smart quotes for now.

Because the flip-flop operator will only return true or false and will never equal 10, 11, 12, 13, 14 or 15.    You need something like:

if (grep $frequency == $_, 10 .. 15) {print "Yes"}
[download]

use 5.010;
for my $frequency ( 1, 10 .. 17 ){
  print $frequency, ' ';
  if ($frequency ~~ [ 10 .. 15 ]) {print "Yes"}
  print "\n";
}
__END__
1
10 Yes
11 Yes
12 Yes
13 Yes
14 Yes
15 Yes
16
17
[download]

use Acme::Tools;
if (between($frequency, 10, 15) {print "Yes"}
[download]

if ($frequency == 10 .. 15) {print "Yes"}
[download]

if ( ($frequency == 10) .. 15 ) {print "Yes"}
[download]

Update: add another example...

If you set it to 10, you will get a Yes:

my $frequency = 10;
if ($frequency == 10 .. 15) {print "Yes"}
[download]

The  .. operator in scalar context is the flip-flop operator, not the range operator.

The .. operator in scalar context is the flip-flop operator, not the range operator.

Firstly, I never explicitly named the .. operator in my post; I merely linked to the Perl official documentation which describes the .. operator, titled Range Operators.

Secondly, the documentation appears to disagree with your assertion that .. in scalar context is not the range operator (emphasis is mine):

In scalar context, ".." returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each ".." operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation, as in sed, just use three dots ("...") instead of two. In all other regards, "..." behaves just like ".." does.

You may refer to the operator however you wish, but it is correct to refer to it as the "range operator" in all contexts, including scalar context.