my $wpi;
BEGIN {
    if ($^O =~ m#MSWin#) {
        require Win32::Process::Info;
        $wpi = Win32::Process::Info->new;
    }
}

# code

my $size;
if ($wpi) {
    $size = ($wpi->GetProcInfo( $pid ))[0]->{'WorkingSetSize'}/1024;
} else {
    open( my $ps,"ps -o rss= -p $pid |" )
     or die "Could not ps -o rss= -p $pid: $!\n";
    while (<$ps>) {
        $size = $1 if m#(\d+)#;
    }
}