See explanation by almut - while() is testing true/false of the statement, not status of <IN>. Here is small mod to your code that avoids the problem.

#!/usr/bin/perl
use warnings;
use strict;

my $content;

open(IN, "<", $ARGV[0]) or die $!;
while ( <IN> ) {
  print $content.= $_;
};
close(IN) or die $!;
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