$this is being assigned the value of a hashref (see perlref), and is NOT a loop. It is the same as doing: 
my %thisHash = (
        _server => undef,
        _database => undef,
        _hSession => undef,
        _hDatabase => undef,
);
my $this = \%thisHash;

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In both cases (yours and the above), the "variables in this loop" (again, note it is not a loop) are the default key-value pairs of the hash (see perldata).

As for the method sub new { ... } and the bless $this, $type, this is a example of a common way to write a constructor method for an object. For more information, start with perlobj and also there is a specifical Object-Oriented Perl section in the Tutorials here on pm.

In reply to Re: Help understanding object constructors by davidrw
in thread Help understanding object constructors by nisha

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