Since you won't provide test data, I'll claim this is untested :)

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1174798

use strict;
use warnings;

# read in mapping hash

my %iphash = split ' ', do {local (@ARGV, $/) = 'file_2'; <>};

# construct pattern to match keys of mapping hash

my $ips = join '|', map quotemeta, sort keys %iphash;
my $pattern = qr/\b($ips)\b/;

@ARGV = 'file_1';

while(<>)
  {
  s/$pattern/$iphash{$1}/g; # do the mapping anywhere in the line
  print;
  }
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