#!/usr/bin/perl
use strict;
use warnings;
my $first = "single_records.txt"; 
my $second = "multiple_records.txt";

open (my $fh, '<', $second) || die "Failed to open $second : $!" ## always check for success on fh
my %second_records = ();
while (<$fh>){
  my $line = $_;
  my ($id,) = split /\s+/, $line; ## capture id
  push @{ $second_records{$id} }, $line;
}
## tidy up
close $fh || die "Failed to close $second : $!";
## open single record file and compare
open ($fh, '<', $first) || die "Failed to open $first : $!";
while (<$fh>){
  my $line = chomp( $_ );
  my ($id,) = split /\s+/, $line; ## capture id
  if ( exists$second_records{$id} ){
    print (join '', "$line matches records:\n", @{ $second_records{$id} });
  }
}
close $fh || die "Failed to close $first : $!";