use strict;
use warnings;

my $string1 = 'amayaM';
my $string2 = 'amayamAn';

my $result = firstStrDiff( $string1, $string2 );

print $result, "\n";

sub firstStrDiff {
	my ( $s1, $s2 ) = @_;

	( $s1 ^ $s2 ) =~ /[^\x00]/;
	substr( $s2, $-[0], 0 ) = '+' if defined $-[0];
	return $s2;
}