use Proc::Background;
my $proc1 = Proc::Background->new($command, $arg1, $arg2);
while( 1 ){
    sleep 1;
    my $newsize = -s $fhorfilename;
#    if( ( $newsize - $oldsize ) > $minimum ){
    if( ( $newsize - $oldsize ) > $buffersize ){
        
    }
    my $readed = read $fh, $buffer, $buffersize;
    doStuff( $readed , $fh, $buffer, $buffersize );
    ...
    last if not $proc1->alive;
}