Single vs Double quotes.

#!/usr/bin/perl

use strict;
use warnings;
use feature 'say';

my $subject = 'Citizens, not subjects!';

sub wrong{

  my ( $emailsubject ) = @_;

  say 'Email subject: $emailsubject';
}

sub right{

  my ( $emailsubject ) = @_;
  say "Email Subject: $emailsubject";

}

wrong( $subject );
right( $subject );
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Strings in Perl: quoted, interpolated and escaped.