use File::stat;
open( my $FH, "<", $fileOrPipeOrWhatever )
  or die $!;
my $fileSize = 0;
my $blockSize = stat($FH)->blksize;
my $len;
while($len = sysread($FH, my $buf, $blockSize) ) {
  $fileSize += $len;
} # while
die $! unless defined $len;
close( $FH );