Your first instinct was correct in both cases. To get named arguments out of the argument array, use my %params = @_;. To pass the arguments to another method in your constructor, do

sub new { 
    my $this = shift;
    $this->_init( @_ );
}
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It doesn't really matter, but Perl convention is to use $self instead of $this. It's worth using if you ever plan to have other Perl people look at your code.