You need to quote them explicitly so they are never tested as numbers. The fat comma stringification comes after numeric interpretation and before keyword recognition. I believe the details should be in toke.c in the root of the perl source distrubution.

(Added) Another few examples,

$ perl -Mstrict -we'BEGIN {$_="foo";} sub bar () {"quux"} my %foo = ( 
+1e0 => "foo", bar => "quux", length => "baz");print "@{[keys %foo]}",
+ $/'
length bar 1
$
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Am I correct in assuming the reason for doing the numeric interpretation first is to be able to say  my %foo = ( 3+5 => "something");? Presumably if fat comma stringification came first, this would become a parse error.

That comes from the high precedence of '+' over '=>'.

If stringification came first there would be no error; you'd wind up with the string '3+5' as a key, which is perfectly all right. Fat comma stringifies like qw, breaking on whitespace.

$ perl -e'%foo = ("3+5", "something");print keys(%foo),$/'
3+5
$
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After Compline,
Zaxo

I don't see why you need to look for reasons for stringification to apply to things that are already valid constants. v-strings introduced a new class of these, and %foo = (v48 => "zero?"); suddenly changed meaning, so that a key of "0" was used instead of a key of "v48". This was "fixed" in 5.8.1 to restore the "v48"-behavior for backward compatilibity, but the 5.6-5.8.0 way (where v48 is "0") was actually more consistent.

Of course, what might otherwise be thought of as a "word" in other contexts in perl would be something like: /^[a-zA-Z_]\w*$/... but the fact that a leading dash is an acceptible part of a "word" with respect to the => greater operator... who can really say?

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The leading dash isn't recognized as part of a bareword by =>, it is actually an alternative method for auto-quoting a bareword. Compare:

$ perl -we'use strict; @_ = (-foo, "bar")'

$ perl -we'use strict; @_ = (foo, "bar")'
Bareword "foo" not allowed while "strict subs" in use at -e line 1.
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Absolutely correct! Wow, I'd somehow completely missed that. From perlop:

Unary "-" performs arithmetic negation if the operand is numeric. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that -bareword is equivalent to "-bareword".

Which leads to this funness:

[me@host]$ perl -le 'use strict; print -foo; print -"-foo"; print -"+f
+oo";'
-foo
+foo
-foo
[me@host]$
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Just when you think there's nothing new to learn.

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That is really neat - I'd assumed -bareword was just a convention and that => was responsible for the quoting. Is there a reason that - and => aren't mentioned in the "Quotes and Quote-like Operators" section of perlop?

Second, is there even a practical solution?

qw can be convenient in these cases:

%a = qw( 09 a );
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  %h = ( 1.000 => 'foo' ) ;
  
  print "value: $h{1}\n" ;
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  $h{2.000} = 'bar' ;
  
  print "value: $h{2}\n" ;
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From perlop: The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. As of release 5.001, it also forces any word to the left of it to be interpreted as a string.

I'd call 01 a word, especially since 01 =~ /^\w+$/ will match. But yup, since perl defines Perl, 01 isn't a word ;)

Hmmm, that looks to me like a documentation bug; IFAIK => quotes identifiers; barewords matching /^[a-z_]\w*$/

Note that in perl versions 5.6.0 - 5.8.0 single-number vstrings are an exception, ie. v80 => means "P",. Perl 5.8.1 reverted this to meaning "v80", again.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

Maybe perlop need some updates!

Graciliano M. P.
"Creativity is the expression of the liberty".

Hahaha, nothing impractical at all. Guess what I ended up doing just to get the code working? I just thought that => autostringification is pretty handy, and that this was a case where perl didn't DWIM.

: Second, is there even a practical solution?

yes, quite a few:

%a = ( '09' => 'a' );
%a = ( '09', 'a' );
%a = qw( 09 a );
$key = '09'; $value = 'a'; %a = ( $key => $value );
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I was more asking if there was a practical way to hack perl so that 09=> would be equivalent to '09'. That said, '09' was in fact the solution I chose to get my code working.