#! perl -slw
use strict;

my $sharePath = '...';
while( 1 ) {
    for( glob $sharePath . "IN/*" ) {
        my( $filename, $expected ) = m[/([^/]+)\.(\d+)$];
        if( $expected == -s ) {
            rename $_, "$sharePath/$filename";
            system 1, 
qq[theProcessingScript.pl <$sharePath/$filename  >$sharePath/OUT/$filename];
        }
    }
    sleep 1;
}