#!/usr/bin/perl -w
use strict;

my $FILE1 = <<END;
ID1     50
ID2     60
ID3     100
END

my $FILE2 = <<END;
ID1     20
ID2     100
ID3      10
END

my %ids;

foreach my $file (\$FILE1, \$FILE2) #just put path name of
                                    #FILE1 and FILE2 here.
                                    #This ref is special because of 
                                    #putting the file contents within
                                    #the code.
                                    #FILE1 and 2 are "hereis" docs.
{
   open (FILE, "<", $file) or die "unable to open $file for read $!";
   while (<FILE>)
   {
      chomp; # delete trailing \n
      
      # here I split on one or more space characters,
      # A tab char doesn't show up well on this forum's text
      
      my ($id, $value) = split (/\s+/, $_);
      push @{$ids{$id}}, $value;
   }
}

#Each key of the hash of %ids contains a reference to
#an array of id's. This is called a HoA - Hash of Array

foreach my $id (sort keys %ids)
{
   print "$id @{$ids{$id}}\n";
}

#This code will run very fast because each line is
#only read one time - Input/Output (I/O) is very 
#"expensive" 

__END__

OUTPUT:
ID1 50 20
ID2 60 100
ID3 100 10