s///e is great when you're playing around with Filter::Simple:

package Filter::ISA;
use strict;
use warnings;

use Filter::Simple;

FILTER_ONLY code => sub {
  s/package\s+(\w+?)\s+isa\s+(.+?);/"package $1; use base qw( " . join
+(" ", split(",", $2)) . " );"/eg;
};

1;
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Usage:

use Filter::ISA;

package derived isa base1, base2;
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