$string =~ s/(?<!Y)(?<!^)X//g;

##</code><code>##

my $rstr = reverse $string;
$rstr =~ s/X(?!Y|\Z)//g;  # note \Z, not $
$string = reverse $rstr;

##</code><code>##

# old:
# $string =~ s/([^Y])X/$1/g;
# new:
$string =~ s/[^Y]\KX//;