The docs clearly state that parameters can not be bare scalars (unless they are file paths, which is not the case for you). Try this:

use IO::Compress::Gzip qw(gzip $GzipError) 
# other code...
IO::Compress::Gzip::gzip(\$content, \$newContent) or die "gzip failed:
+ $GzipError\n"
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Also, you're doing the file slurp in a wrong and slow way:

    my @transferFileContents = <FILE_TO_TRANSFER>;
    foreach(@transferFileContents)
    {
      $content .= $_;
    }
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It is possible to write this as:

$content = do { local $/; my $rv = <FILE_TO_TRANSFER>; $rv };
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which means:

$content = do { # minimize scope
    local $/; # make input separator undef 'till the end of this scope
    my $rv = <FILE_TO_TRANSFER>; # get all contents
    $rv # return it
};
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