Create a hash. Read the 10K file first. Use the 1st col as the key and the rest of the record as the value.

Then when you loop over the second file if the first col exists in the original hash, write whatever record you want to the file.

Here is some code - It may not do exactly what you want, but that is because I'm guessing your spec.

## UNTESTED
use strict;

my %hash;
open(FH,'<','oldfile') or die "$!\n";
foreach (<FH>) {
  chomp;
  my ($key,$data) = split(/\|/,$_,2);
  $hash{$key} = $data;
}
close FH;

open(NEW,'<','newfile') or die "$_\n";
open(OUT,'>','outfile') or die "$_\n";

foreach (<NEW>) {
  chomp;
  my ($key) = split(/\|/,$_,2);
  if ( exists $hash{$key} ) {
    print OUT "$hash{$key}\n";
  }
}
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