The version not relying on File::Slurp would be the following:

use Tie::File;

use strict;

my $file1 = 'file1.txt';
my $file2 = 'file2.txt';

open my $fh, $file2 or die "Can't open $file2: $!";
my %hash = map {
    chomp;
    my @data = split /\s+/;
    $data[0] => $data[2];
} <$fh>;

tie my @array, 'Tie::File', $file1 or die "Can't open $file1: $!";

foreach (@array) {
    s{^(\S*\s+)(\S+)}{
        if (exists $hash{$2}) {
            print "Replacing $2 with $hash{$2}\n";
            $1 . $hash{$2};
        } else {
            print "Can't find $2, not changing\n";
            $1 . $2;
        }
    }e;
}
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- Miller