# open multiple filehandles at the same time
# copy content to another file
open (SOURCEFILE, "mysubdir/myinputfile.txt") || die; 
open (DESTFILE, ">mysubdir/myoutputfile_2.txt") || die;	
	@contents = <SOURCEFILE>;
	print DESTFILE @contents;	
close SOURCEFILE; 
close DESTFILE;


open (SOURCEFILE, "mysubdir/myinputfile.txt") || die; 
open (DESTFILE, ">mysubdir/myoutputfile_3.txt") || die;	
	while (defined ($line = <SOURCEFILE>)) {
	#you can do work here to update the line
	$line = uc $line;  #uppercase
	print $line;
	print DESTFILE $line;
}	
close SOURCEFILE; 
close DESTFILE;

#three free file handlers <STDIN> <STDOUT> <STDERR> 
#by default, connect to terminal