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;