open(my $master_fh => "master.file") or die "ack: $!";
open(my $subset_fh => "subset.file") or die "ack: $!";

my %subset = map { chomp; split ':', $_, 2 } <$subset_fh>;

open(my $result_fh => ">", "result.file") or die "ack: $!";

while(<$master_fh>) {
  my($key, $rest) = split ':', $_, 2;
  print {$result_fh} join ':', $key, $subset{$key}, $rest
    if exists $subset{$key};
}