use strict ;
use warnings ;

my $mock = "I\nII\nIII\nIV" ;
open my $FH, '<', \$mock ;

$_ = 'firstborn' ;

while (local $_ = <$FH>) {
  chomp ;
  print "$_=" ;
  tr/IV/iv/ ;
  amuck() ;
  print "$_ " ;
} ;

if (!defined($_)) { $_ = '*undef*' ; } ;
print "  -- $_\n" ;

sub amuck { $_ = '!!!' } ;