my $Usage = "Usage: $0  file1  file2 > combined.file\n";
die $Usage unless ( @ARGV == 2 and -f $ARGV[0] and -f $ARGV[1] );

my ($inf1,$inf2) = @ARGV;

##</code><code>##

open( IN1, $inf1 ) or die "Can't open $inf1: $!";

##</code><code>##

my %file1;

while (<IN1>) {
    next unless (/.:./);  # maybe warnings aren't needed here
    chomp;
    my ($key,$val) = split( /:/, $_, 2 );
    if ( $key =~ /^\s*$/ ) {
        warn "$inf1:$.:Empty key field\n";
        next;
    }
    if ( exists( $file1{$key} )) {
        warn "$inf1:$.:Duplicate key string\n";
        next;   # might want to say more about that
    }
    # might want to check $val too...

    # get here if everything's okay:
    $file1{$key} = $val;
}
close IN1;

##</code><code>##

open( IN2, $inf2 ) or die "Can't open $inf2: $!";
while (<IN2>) {
    next unless ( /.:./ );
    my ($key,$val) = split( /:/, $_, 2 );
    # error checking similar to what was done for $inf1
    # ...

# if $key did not occur in $inf1, and if this means that you
# don't want to list it in the final output, then don't put 
# it into the hash in the first place
    unless ( exists( $file1{$key} )) {
        warn "$inf2:$.:Key $key not found in $inf1\n";
        next;
    }
    $file1{$key} .= ";$val";
}
close IN2;

##</code><code>##

  perl_script  infile.a infile.b > outfile  2> script.errs