foreach $string (
  "s005219/Doe John|John.Doe\\",
  "s0052194/Doe Thomas|Thomas.Doe\\",
) {
   ($reversed_name) = $string =~ m!/(.*?)\|!;
   print($reversed_name, "\n");
}

# Prints
# ======
# Doe John
# Doe Thomas

##</code><code>##

foreach $string (
  "s005219/Doe John|John.Doe\\",
  "s0052194/Doe Thomas|Thomas.Doe\\",
) {
   if (m!/(.*?)\|!) {
      print("$1\n");
   }
}