Bad news: I can't find any existing module to do this.

Good news: Win32::API can be used to access all system calls and any DLL using only Perl and no compiler. The system call in question appears to be LockWorkStation.

Update: It might be as simple as the following (untested) code:

use Win32::API ();
Win32::API->Import("user32", "BOOL LockWorkStation()");
LockWorkStation();
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Update: Here's a tested version with error checking:

use Win32::API ();

Win32::API->Import("user32", "BOOL LockWorkStation()")
   or die("Unable to import LockWorkStation: $^E\n");
    
LockWorkStation()
   or die("Unable to lock workstation: $^E\n");
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If Import returns false and $^E is numerically equal to 127 ("The specified procedure could not be found"), then it's probably because the function is not required on that particular OS.

Maybe the following line could prove to be useful too.

%windir%\system32\rundll32.exe user32.dll, LockWorkStation
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