$x =~ /Y[A-Z]\d+[^\d\\]*$/

##</code><code>##

for  my $x ("\\\\files\\builds\\data\\M9998SBQCACSYD30401S", "\\\\files\\builds\\data\\M9998SBQCACSAD30401S") {
    if($x =~ /Y[A-Z]\d+[^\d\\]*$/) {
		print "$x matches\n";
	} else {
		print "$x doesn't match\n";
    }
}

##</code><code>##

\\files\builds\data\M9998SBQCACSYD30401S matches
\\files\builds\data\M9998SBQCACSAD30401S doesn't match