...and you can also be defensive and protect yourself from other code that isn't as nice:

sub mine {
    local( $_ );
    while(  <FILE>  ) {
        s/this/that/g;
        {
            local( $_ )= $_;
            yours( $_ );
        }
        s/that/this/g;
    }
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though if you find yourself doing that I strongly suggest you just switch to using my $var instead of $_.

        - tye (but my friends call me "Tye")