What is the pipe symbol doing to my array?

Hi All,

I see what is happening, but exactly what I am not sure:

[0] > my @x="a","b","c","d";
[a b c d]

[1] > say @x
[a b c d]

[1] > say |@x
abcd


I ask because I want to know its effect is on
     my $proc = run(|@x, :err, :out)


Many thanks,
-T