Your code creates an infinite loop, though, if $self->{$k}->{blahblah} never evaluates to true. (whereas the OP's falls through to "return undef;")

perl -Mstrict -lw
my %self;
$self{$_}{blahblah} = 0 for qw/1 2 3/;
while (my $k = each %self || each %self) {
    print "testing:$k";
    print "true for $k" and last if $self{$k}{blahblah};
}
__OUTPUT__
testing:1
testing:3
testing:2
testing:1
testing:3
testing:2
testing:1
... etc. ...
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