Either you expect the code that uses in the file handle in the if (causing the error message to be far away from the error check), or you just created a file handle that gets closed before you get a chance to use it. Slight reorganization of your code:

if ( $outfile !~ m{\A(\w+)\z} )
{
    die qq{Disallowed characters in filename\n};
}

$outfile = $1;
open my $outFH, q{>}, $outfile
   or dir qq{open: $outfile: $!\n};
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