sub ReadTheEntireFile
{
  my $F = defined $_[0] ? $_[0] : '';
  $F =~ tr`<>*?\"\|\x00-\x1F``d;      # Remove illegal characters.
  length($F) or return '';            # Missing file name?
  -e $F and -f $F or return '';       # Make sure file exists.
  my $SIZE = -s $F;                   # Get file size.
  $SIZE or return '';                 # Return zero bytes.
  my $BUF = '';                       # Read buffer
  local *FILE;
  sysopen(FILE, $F, 0) or return '';  # Open file for read only.
  my $L = sysread(FILE, $BUF, $SIZE); # Read the entire file
  close FILE;
  return $BUF;
}
[download]

Again, you don't need both file tests to know if a file exists.

$L contains the amount of data returned from the file. You should compare that value to $SIZE to verify that the entire file has been returned.