To prove it to myself ...

#!/usr/bin/perl -w

use strict;
use warnings;

my $d = "This is a double-quote string\n";
my $s = 'This is a single-quote (apostrophe) string' . "\n";

print $d;
print $s;

# Results ...

BEGIN { $^W = 1; }
use warnings;
use strict 'refs';
my $d = "This is a double-quote string\n";
my $s = "This is a single-quote (apostrophe) string\n";
print $d;
print $s;
quotes.pl syntax OK
[download]

Looks pretty convincing to me!

Very true. I was going to mention that myself.

Still i believe a benchmark always says something. It's like profiling; it is often important to think about using it. Even if the benchmark tells nothing in a case like this one, still, the effort of thinking about why it does not tell anything is a useful exercise.