use IO::Handle;

open INPUT,  '<', "input.txt"  or die $!;
open OUTPUT, '>', "output.txt" or die $!;
open ERROR,  '>', "error.txt"  or die $!;

 STDIN->fdopen( \*INPUT,  'r' ) or die $!;
STDOUT->fdopen( \*OUTPUT, 'w' ) or die $!;
STDERR->fdopen( \*ERROR,  'w' ) or die $!;

# prints to output.txt:
print "Hello Output File\n";

# reads everything from input.txt and prints it to output.txt:
print <>;

# prints to error.txt:
warn "Hello Error File\n";