Update: After talking with ImpalaSS, modified code to handle multiple entries in the first file with the same matching key.

---

Quick solution. Not the best way, as it is not very flexible, but gets the job done. Requires the two files to be joined as the first and second arguemnts, and the file to be created as the third. Outputs any "unmached" records.

#!/dev/null/perl -- :)

use strict;

my $first  = shift or die "Need the first file!\n";
my $second = shift or die "Need the second file!\n";
my $third  = shift or die "Need the third file!\n";

## May as well test them all now
open(FIRST,  "$first")  or die "Could not open $first: $!\n";
open(SECOND, "$second") or die "Could not open $second: $!\n";
open(THIRD,  ">$third") or die "Could not write $third: $!\n";

my (%first, %found);
while(<FIRST>) {
  my ($key, @cols) = split(m#\|# => $_, -1);
  push(@{$first{$key}}, \@cols);
  $found{$key} = $.;
}
close(FIRST);

while(<SECOND>) {
  my ($one, $key, @cols) = split(m#\|# => $_, -1);
  if (exists $first{$key}) {
    delete $found{$key};
    for (@{$first{$key}}) {
      print THIRD "$_->[0]|$one|$cols[0]\n";
    }
  }
  else {
    printf "Line %5d: Record %5d exists in $second but not in $first\n
+",
           $.,$key;
  }
}
close(SECOND);
close(THIRD);

## Double check the first file:
for (sort {$found{$a} <=> $found{$b}} keys %found) {
  printf "Line %5d: Record %5d exists in $first but not in $second\n",
         $found{$_},$_;
}
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