First, obvious from my text: I asked for a portable way.

All right, create a sub called "KillProc()" and then put an if statement in that sub. Execute block A if it's Linux, or execute block B if it's Windows. Portable. Done!

A command which only works on Windows doesn't help.

Why? Can't you detect the current running OS from your Perl script and put an if statement into your program? Is this against your religion? WHATEVER solution you are looking for, if you look deep down into it, somewhere at the deep inside a module, you'll find an if statement that executes two different set of codes for Linux and Windows, because the two don't work the same!

It's not hard. I have done this many times. For example:

sub CLS
{
  $^O =~ m/MSWIN/i and return system('CLS');
  $^O =~ m/DOS/i and return system('COMMAND.COM /C CLS');
  $^O =~ m/LINUX/i and return print "\x1Bc\x1B[0m\x1B[3J\x1B[H\x1B[2J"
+;
  print (($^O =~ m/DARWIN/i) ? "\x1B[3J" : ' ' x 4000);
}
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So, you want to kill one specific task...

C:\>TASKLIST

Image Name                   PID Session Name     Session#    Mem Usag
+e
========================= ====== ================ ======== ===========
+=
System Idle Process            0                         0         28 
+K
System                         4                         0        236 
+K
smss.exe                     456                         0        384 
+K
csrss.exe                    508                         0      3,320 
+K
winlogon.exe                 548                         0      1,348 
+K
SERVICES.EXE                 592                         0      3,752 
+K
LSASS.EXE                    604                         0      1,312 
+K
SVCHOST.EXE                  776                         0      3,448 
+K
SVCHOST.EXE                  820                         0      3,848 
+K
SVCHOST.EXE                  888                         0     14,340 
+K
SVCHOST.EXE                  932                         0      4,560 
+K
SPOOLSV.EXE                 1032                         0      4,132 
+K
SVCHOST.EXE                 1096                         0      3,276 
+K
EXPLORER.EXE                1396                         0     27,320 
+K
SVCHOST.EXE                 1548                         0      3,796 
+K
mypal.exe                    696                         0    194,636 
+K
Notepad2.exe                1540                         0      5,984 
+K
cmd.exe                     1588                         0      2,332 
+K
wmiprvse.exe                1840                         0      5,692 
+K
tasklist.exe                1152                         0      4,100 
+K

C:\>TASKKILL /F /PID 1540
SUCCESS: The process with PID 1540 has been terminated.

C:\>
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It's that simple!