I'd suggest avoiding the file system completely and using a Named Pipe.

Your perl code creates a named pipe before starting the child process, then supplies the full name of the named pipe to the (asynchronously started) child process as the password file name; it then waits for the child to open the "file" and supplies the password.

Using Win32::Pipe this can be done so:

#! perl -slw
use strict;
use Win32::Pipe;

my $pipe = Win32::Pipe->new( 'MyPipe' );

print system 1, 'c:\test\dummyCapp.exe \\\\.\\pipe\\MyPipe' or die $?;

$pipe->Connect;

$pipe->Write( 'The quick brown fox' );

$pipe->Disconnect;

$pipe->Close;
[download]

I knocked up this as a substitute for the sshg3 app:

#include <stdio.h>

int main( int argc, char **argv ) {
    FILE *f;

    if( argc < 2 ) {
        fprintf( stderr, "No filename given\n" );
        exit( -1 );
    }

    if( f = fopen( argv[1], "r" ) ) {
        int read = 0;
        char pword[ 1024 ];

        if( read = fread( pword, sizeof( char ), 1024, f ) ) {
            printf( "Got: '%s'\n", pword );
        }
        else {
            fprintf( stderr, "Failed to read anything\n" );
            exit( -2 );
        }
    }
    else {
        fprintf( stderr, "Couldn't open file %s: %d\n", argv[1], GetLa
+stError() );
        exit( -3 );
    }
    printf( "Ending...\n" );
    return 0;
}
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And running the perl script gives:

C:\test>junk50
560
Got: 'The quick brown fox'
Ending...
[download]