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.