Discussed at some length in Curious Constructor Syntax.

The short answer is that new returns a newly blessed object by convention. You could as easily write something that doesn't always return a new object. Consider:

package Singleton;
...
my $singleton = undef;

sub new {
    my $pkg = shift;
    $singleton ||= bless {}, $pkg;
}
...
my $frob = new Singleton();
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