my %seen;
open(B, "brd_sym_pn.txt") or die "...";
while (<B>) {
  my ($RefDes, $Pnumm, $Pkg_Type) = ...parse these from the line...
  $seen{$RefDes, $Pnumm, $Pkg_Type} = 1;
}
close(B);

open(S, "sym_text_latest.txt") or die "...";
while (<S>) {
  my ($RefDes, $Pnumm, $Pkg_Type) = ...parse these from the line...
  $seen{$Refdes, $Pnumm, $Pkg_Type} += 2;
}
close(S);

while (my ($key, $val) = each %seen) {
  if ($val == 1) {
    # $key is in first file but not second
  } elsif ($val == 2) {
    # $key is in second file but not first
  } else {
    # key is in both files
  }
}