# names.txt
1000:John Smith
2000:Jane Doe
3000:John Doe
4000:George Bush
5000:Saddam Hussein

# ids.txt

blah blah blah blah -- this line contributed by 1000
foo bar baz etc etc contributed by 4000, my old friend
once upon a time thanks to 3000 for his help
it was a dark and stormy night, wrote 1000
this line written by both 5000 and 2000

Script:

use strict;

my ($id,$name,$number,%database);
open(NAMES,'names.txt') || die "$!";
while(<NAMES>){
chomp($_);
($id,$name) = split(':',$_);
 $database{$id} = $name;
}

close(NAMES);
open(IDS,'ids.txt') || die "$!";
open(IDSREPLACED,'>ids-replaced.txt') || die "$!";

while(<IDS>){
 foreach $number (keys(%database)){
		$_ =~ s/$number/$database{$number}/g;
 }
 print IDSREPLACED $_;
}

# produces

blah blah blah blah -- this line contributed by John Smith
foo bar baz etc etc contributed by George Bush, my old friend
once upon a time thanks to John Doe for his help
it was a dark and stormy night, wrote John Smith
this line written by both Saddam Hussein and Jane Doe