open MASTER, "file3" or die "cant't open master file: $!\n";
open SUBSET, "file2" or die "can't open subset file: $!\n";
open OUT, ">outfile" or die "can't open output file: $!\n";
while ( my $sline = <SUBSET> ) {
    chomp $sline;
    my ( $subkey, $subvals ) = split /:/, $sline, 2;
    my ( $mkey, $mvals );
    while ( my $mline = <MASTER> ) {
        chomp $mline;
        ( $mkey, $mvals ) = split /:/, $mline, 2;
        last if $mkey eq $subkey;
    }
    print OUT join ":", $subkey, $subvals, $mvals;
}
close SUBSET;
close MASTER;
close OUT or die "couldn't close outfile: $!\n";