Here's a command line driven Inline::C script. Supply the pid and required priority (idle/below/normal/above/high/realtime) as arguments. (Converting numeric priorities back to text for output left as an exercise.):

#! perl -sw
use 5.010;
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'Priority', CLEAN_AFTER_BUILD => 0
+;

HANDLE openProcess( SV *pid ) {
    HANDLE ph;
    if( !( ph  = OpenProcess( PROCESS_ALL_ACCESS, 0, SvIV( pid ) ) ) )
        croak( "Failed to open process pid:%d (See $^E)\n", SvIV( pid 
+) );
    return ph;
}

SV *setPriorityClass ( SV *pid, SV* class  ) {
    HANDLE ph = openProcess( pid );
    if( !SetPriorityClass( ph, SvUV( class ) ) )
        croak( "SetPriorityClass failed; See $^E \n" );
    return class;
}

int getPriorityClass ( SV *pid ) {
    HANDLE ph = openProcess( pid );
    DWORD class;
    if( !( class = GetPriorityClass( ph ) ) )
        croak( "SetPriorityClass failed; See $^E \n" );
    return class;
}

END_C

use constant PRIORITY => {
    IDLE => 0x40, BELOW => 0x4000, NORMAL => 0x20,
    ABOVE => 0x8000, HIGH => 0x80, REALTIME => 0x100,
};

my $old = eval{
    getPriorityClass( $ARGV[ 0 ] )
} or die $^E;
my $new = eval{
    setPriorityClass( $ARGV[ 0 ], PRIORITY->{ uc $ARGV[ 1 ] } )
} or die $^E;
say "Priority of process id: $ARGV[ 0 ] changed from $old to $new";
__END__
C:\test>priority 1460 idle
Priority of process id: 1460 changed from 32 to 64

C:\test>priority 1460 below
Priority of process id: 1460 changed from 64 to 16384

C:\test>priority 1460 normal
Priority of process id: 1460 changed from 16384 to 32

C:\test>priority 1460 above
Priority of process id: 1460 changed from 32 to 32768

C:\test>priority 1460 high
Priority of process id: 1460 changed from 32768 to 128

C:\test>priority 1460 realtime
Priority of process id: 1460 changed from 128 to 256
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