#-- p1 not changed on disk ;-)
use File::Slurp;
my $text = read_file( 'p1.pm' ) ;
$text =~ s/^\s*my(\s+\$var1)/our$1/ms;
eval $text;
#-- above not recommended!

package p2;

#  use p1;

our @ISA=qw(p1);

sub set_var1 {
    my $self = shift;
    my $new_var1 = shift;
#   $var1 = $new_var1;
#   return $var1;
    $p1::var1 = $new_var1;
    return $p1::var1;
}

1;