You could use a compiled regexp:

perl -E '
my $x = "foo+bar+baz";
my $y = "+";
my $z = qr/\Q$y\E/;
say for split $z, $x;
'
[download]

Output:

foo
bar
baz
[download]

... but why not just use:

say for split /\Q$y\E/, $x;
[download]

in all cases? (There's probably a reason; I don't know it.)

Hope this helps!