use strict;


my $SetName = "MR_\\1\\2\\3";
my $orig1 = "P";
my $orig2 = "A";
my $orig3 = "T";
my $new1 = "\\\\3";
my $new2 = "\\\\2";
my $new3 = "\\\\1";

if ($new1 =~ /\\3/) {
	if ($SetName =~ /\\3/) {
		print "hi!! 3\n";
		$SetName =~ s/$new3/$orig1/;
	}
	if ($SetName =~ /\\2/) {
		print "hi!! 2\n";
		$SetName =~ s/$new2/$orig2/;
	}
	if ($SetName =~ /\\1/) {
        print "hi!! 1\n";
		$SetName =~ s/$new1/$orig3/;
	}
	print "SetName = $SetName\n";
} 

The output is:
Hi!! 3
Hi!! 2
SetName = MR_PA\3