An alternative approach would be to use this C code and some XS magic, or Inline::C. Assuming, of course, that getting a DOSsy perl and C to talk to each other is practical:

/* Call the set() function with two pointers to char as the
 + parameters.  The first points to the environment variable
 + NAME, the second to the VALUE.  The NAME is made
 + upper-case automatically.
 */

#include <dos.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <conio.h>

#define VLEN 80
#define BUFLEN 1000

unsigned int _psp;
unsigned short peekw();
unsigned char peekb();
void pokeb();

set (char *varname, char *value)
  { unsigned short x, p, y, envsize, free;
    int found;
    char  c, var[VLEN+BUFSIZ+1];
    int i, varlen;

    varlen = strlen(varname);
    for (i=0; i <= varlen; i++) var[i] = toupper(varname[i]);

    strcat(var,"=");
    varlen++;

    p = _psp;

    do
      { x = p;
        p = peekw(x,0x16);
      } while (p != x);

    if (peekb(x-1,0) != 0x4d)
      { cputs("Wrong DOS segment - weirdness is occuring - PANIC!\n\01
+5");
        exit(2);
      }

    p = peekw(x,0x2c);
    envsize = peekw(p-1, 3) << 4;

    x = 0;
    found = 0;
    do
      { if (mystrncmp(p,x,var,varlen) == 0)
        { y = x;
          found = 1;
        }
        while (c=peekb(p,x))
          { if (found == 2)
            { pokeb(p,y,c);
              y++;
            }
            x++;
          }

        switch (found)
          { case 1: found=2; break;
            case 2: pokeb(p,y,c); y++; break;
          }

      } while (peekb(p,++x));

    if (found)
      { pokeb(p,y,'\0');
        x = y;
      }

    free = envsize - x - 1;

    for (i=2; i < strlen(value) + 2; i++) value[i] = tolower(value[i])
+;

    strcat(var,value);

    y = strlen(var);
    if (y+1 > free)
      { cputs("Sorry, your environment space is full, program abort.\n
+\015");
        cputs("Try \"shell=c:\command.com /p /e:62\" in config.sys to\
+n\015");
        cputs("increase your environment space to 992 bytes\n\015");
        exit(3);
      }
    for (i=0; i <= y; i++) pokeb(p, x++, var[i]);

    pokeb(p, x, 0);
  }

mystrncmp(seg,offset,source,n)
short seg,offset;
int n;
char *source;
{
  int i, dif;

  for (i=0; i < n; i++) {
    dif = peekb(seg,offset+i) - source[i];
    if (dif) return dif;
  };
  return 0;
}

unsigned char peekb(seg,offset)
short seg,offset;
{
  char far *fptr;
  FP_SEG(fptr) = seg;
  FP_OFF(fptr) = offset;
  return *fptr;
}

void pokeb(seg,offset,what)
short seg,offset;
char what;
{
  char far *fptr;

  FP_SEG(fptr) = seg;
  FP_OFF(fptr) = offset;
  *fptr = what;
}

unsigned short peekw(seg,offset)
short seg,offset;
{
  unsigned far *fptr;

  FP_SEG(fptr) = seg;

  FP_OFF(fptr) = offset;
  return *fptr;
}
[download]

Note that I have only used that with MS Quick C, it may or may not work with other compilers. No, I no longer have any idea how it works. There are probably cut n' paste errors. It may turn your cat into an almond. Caveat Programmer.

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