my $p = "$2$1";       # my $p = $2.$1;
                      # Doesn't change the value of $2 or $1.

##</code><code>##

my $p = "$ARGV[0]";   # my $p = "".$ARGV[0];
                      # Doesn't change the value of $ARGV[0].

##</code><code>##

use String::Interpolate qw( interpolate);

my $t = '$1$2';
  or 
my $t = $ARGV[0];

my $p = interpolate($t);