If I do not explicitly want a specific initial value for a variable, I purposely leave it undefined so that I get a warning if I use it before I was planning to.

If I do not want an explicit initial value for a variable, I tend to set it explicitly to undef. It tends to pair up well with a latter test for definedness, and it also shows that I have considered that the variable containing an undefined value is actually, er, defined behaviour. Consider the snippet:

my $key = undef;
for my $k( keys %h ) {
  $key = $k if $h{$k} eq 'foo';
}
if( defined $key ) {
  # ...
}
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I find it makes it clearer to see that the variable starts out undefined and that the loop could set the variable to some value.

I'm not sure that makes sense. Someone fluent in Perl is supposed to know that my $var; leaves $var undefined. I wouldn't consider the following any less readable than your code - or maybe, vice versa, don't consider your code any more readable than the following:

my $key;
for my $k( keys %h ) {
  $key = $k if $h{$k} eq 'foo';
}
if( defined $key ) {
  # ...
}
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It's the same as using $_ - you can of course go and write

while(defined($_ = <STDIN>)) {
    next unless $_ =~ /^something/;
    # ...
}
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but in my opinion, that is, if anything, less rather than more readable. It's more accomodating to a Perl beginner maybe, who would be a bit lost as to what's happening if the idiomatic forms were used, but it's a whole lot more laborous to read too. But that may just be my personal preference.

Makeshifts last the longest.