Since I started using Perl I've always initialized my variables, many times with undef, but I've seen a lot of CPAN code that doesn't. Maybe it's considered wasteful or uncool?

I usually "declare" my variables on first use - or phrased differently, as soon as I "declare" my variables, I use them. Which, for scalar variables, usually means they immediately get a value (unless I just want their reference). But if they get a value, it usually isn't "undef". I find both

my $x;
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and

my $y = undef;
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to be pretty wasteful. I prefer

my $z = shift;
my $w = $hash{foo};
my ($v, $u) = /pattern/g;
while (my ($s, $t) = each %hash) {...}
while (my $r = <>) {...}
...
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