I think your solution is fine. If you want to get fancy, you could use my Tie::Handle::Argv, which is the basis of my File::Replace::Inplace, an emulation of the -i switch (note: Perl v5.16 strongly recommended):

use warnings;
use 5.016;

package Tie::Handle::FancyThingy {
    use parent 'Tie::Handle::Argv';
    use File::Basename qw/fileparse/;
    sub OPEN {
        my ($self, $origfn) = @_;
        my ($fn, $dirs, $ext) = fileparse($origfn, qr/\.[^\.]+$/);
        my $outfn = $dirs.$fn.'.out';
        print STDERR "Debug: $origfn => $outfn\n";
        open ARGVOUT, '>', $outfn or die "$outfn: $!";
        select ARGVOUT;
        return $self->SUPER::OPEN($origfn);
    }
    sub inner_close {
        my $self = shift;
        select STDOUT;
        return $self->SUPER::inner_close(@_);
    }
}

tie *ARGV, 'Tie::Handle::FancyThingy';
while (<>) {
    chomp;
    say translate($_);
}

sub translate { return "<".shift.">" }
[download]

A more simplistic approach might be to just use the -i switch to write the "backup" (input) files to a different directory and then rename the output afterwards.

Minor edit to code: Replaced my $ofh (output filehandle) with the more appropriate ARGVOUT.