Yes, you can call an instance method on $self in the constructor, provided you have first called bless on it (as in your example).

is it bad programming practice ?

No, it’s standard practice. See perlobj:

Once we've blessed the hash referred to by $self we can start calling methods on it. This is useful if you want to put object initialization in its own separate method:

  sub new {
      my $class = shift;

      my $self = {};
      bless $self, $class;

      $self->_initialize();

      return $self;
  }
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HTH, and welcome to the Monastery!