Perl 5 version 26.1 documentation
Returns the next character from the input file attached to FILEHANDLE, or the undefined value at end of file or if there was an error (in the latter case $! is set). If FILEHANDLE is omitted, reads from STDIN. This is not particularly efficient. However, it cannot be used by itself to fetch single characters without waiting for the user to hit enter. For that, try something more like:
Determination of whether
$BSD_STYLEshould be set is left as an exercise to the reader.