You can achieve within a program the same effect as with the -i command line switch, by setting the perlvar $^I to the value of the backup extension. For example, the following snippet upper-cases the contents of the files foo.txt and bar.txt in place, and puts backup copies in foo.txt.bkp and bar.txt.bkp (note that there is no explicit open):

use strict;
use warnings;

$^I = q(.bkp);
@ARGV = qw( foo.txt bar.txt );
print uc while <>;

__END__
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Setting $^I above to the empty string, causes foo.txt and bar.txt to be upper-cased in place, but no backups are created.