# Some time later, changed the code to collect all the input "do { ... };",
# not just the first line of STDIN.

#!/usr/local/bin/perl

use v5.26; use warnings;

use Data::Dumper qw[ Dumper ];
use Getopt::Long;


my $opt;
GetOptions( 'opt:s' => \$opt )
   or die $!;

say Dumper( { 'Initial, opt' => $opt } );

if ( defined $opt )
{
   if ( -p STDIN )
   {
      say q[Any option argument will be abandoned; collecting STDIN ...];
      # Could process standard input line by line.
      # Here, all is collected at once.
      do { local $/; $opt = <STDIN> };
      say Dumper( { ' ...after collecting STDIN, opt' => $opt } );
   }
   elsif ( $opt eq '' )
   {
      die qq[Neither a value was given for \$opt, nor was specified on standard input.];
   }
   else
   {
      say qq[STDIN is not a pipe; using existing \$opt = $opt.];
   }
}
else
{
   say qq[\$opt was not used, so STDIN was ignored.];
}

say Dumper( { 'Final, opt' => $opt } );