#!/usr/bin/env perl
use 5.010; use warnings; use strict;

my %h;
open my $fd2, '<', 'file2' or die $!;
while (<$fd2>){
    chomp;
    my($k, $v) = split /_/;
    $h{$k} = $v;
}

open my $fd1, '<', 'file1' or die $!;
while (<$fd1>){
    s/(\w+)/$h{$1}?"${1}_$h{$1}":$1/ge;
    print;
}