As Kenosis said you're doing too much work. File::Slurp does it all for you:

use File::Slurp 'read_file';

my $string = read_file($filename);
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In the absence of File::Slurp, the best way is:

my $string = do {
   open my $fh, '<', $filename;
   local $/;
   <$fh>;
};
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Or even:

my $string = do { local (@ARGV, $/) = $filename; <> };
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... though that last one can boggle some readers' minds.

You can read about the magic $/ variable in perlvar, but the short story is: $/ is the string that Perl considers to be the line ending when it's reading a file line-by-line; when $/ is undef, Perl will consider the whole file to be a single line, so <$fh> (which normally reads a single line from $fh) will read the whole file.