That's what the start command does unless you use the /B switch. However, trying to exec start to drop the priority of the current process isn't going to work either, which is why I suggested a batch file.

If you want to do it from within the script, you can use Win32::API::Prototype to do it this way:

#! perl -slw
use strict;
use Win32::API::Prototype;;

{
    local $^W = 0; ## Ignore warning from Win32::API::Prototype
    ApiLink( 'kernel32', 'HANDLE GetCurrentProcess( )' ) or die $^E;
    ApiLink( 'kernel32', 'HRESULT SetPriorityClass( HANDLE p, DWORD c 
+)' )
        or die $^E;
}

SetPriorityClass( GetCurrentProcess(), -1 );
print "Check priority for process $$ in the Task Manager";
## You should see that it has been set to 'LOW'
<STDIN>;
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Or using Win32::Process like this:

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

Win32::Process::Open( my $hProcess, $$, 0 ) or die $^E;
$hProcess->SetPriorityClass( THREAD_PRIORITY_IDLE );
undef $hProcess;

print "Check priority of process $$ in Task Manager";
<STDIN>;
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