my $addr = mapFile( ... );
substr( $addr, 32, 4 ) =
    pack 'V', 1+ unpack( 'V', substr $addr, 32, 4 );

##</code><code>##

mapFile( my $addr, ... );
substr( $addr, 32, 4 ) =
    pack 'V', 1+ unpack( 'V', substr $addr, 32, 4 );