# cat check-standard-input-pipe.pl ; perl check-standard-input-pipe.pl ; echo "x\ny"| perl check-standard-input-pipe.pl
#!/usr/local/bin/perl

use v5.36;

-p *STDIN{q/IO/}
   or die qq[No standard input pipe, exiting ...\n] ;

process( *STDIN{q/IO/} );
exit;

sub process( $source )
{
   while ( my $line = <$source> )
   {
      print qq[got: <$line>];
   }
   return;
}

No standard input pipe, exiting ...
got: <x
>got: <y
>