You can set the INPUT_RECORD_SEPARATOR $/ to your $external_info.

$/="$external_info";
open(FH, "/path/to/file") || die "Can't open file: $!";
while (<FH>) {
  if length($_) != (stat FH)[7]) {
    print "great\n";
    last;
  }  
}
[download]

The bad Thing about this way is, that again the whole File goes into memory if the $external_info is not in the File...
So the other solutions might be a bit better ;-)

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