Well, this is pretty miniscule, but you did ask -- you could initialize vars outside of your loop (especially for @fields) to avoid destroying/creating a new variable each time; also, I think handling split() a fixed string '|' instead of a regex might be optimized further:

my ($file, $line, @fields);

foreach $file (@files) {
  open(FILE, $file) or die "Nya, nya: $!\n";
  while($line = <FILE>) {
     @fields = split('|', $line); 
     print OUTPUT join("|", @fields);
  } 
}
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