#!/usr/bin/perl
use strict;
use warnings;

my %intros = ();
open my $fh, '<', 'big_pipe_delimited_file' or die $!;
while (<$fh>) {
  my ($intro) = /^(.*?)\|/;
  $intros{$intro} = 1;
}

my %introsh = ();
for (keys %intros) {
  open $introsh{$_}, '>', 'big_pipe_delimited_file_' . $_ or die $!;
}

seek($fh, 0, 0) or die $!;
while (my $line = <$fh>) {
  my ($intro) = $line =~ /^(.*?)\|/;
  print $introsh{$intro} $line;
}