my %map = (
  (Page) => 'mypage',
  (Index) => 'myindex',
);
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perl-diddler:

In this case, the => operator treats the bare word to the left as a string literal.

...roboticus

Is there anyway I can force it into being eval'ed for the value of constant? (or do I have to put it a variable...?)

#/usr/bin/perl
use strict;
use warnings;

use constant {
  Undefined => 0,
  Page => 1,
  Index => 2,
};

{my %map = (
  Page => 'mypage',
  Index => 'myindex',
);

use Data::Dumper qw(Dumper);
print Dumper \%map;}

{my %map = (
  Page() => 'mypage',
  Index() => 'myindex',
);

print Dumper \%map;}
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And this is why Perl Best Practices says to use Readonly (with Readonly::XS installed, preferably) instead of constant.

Well, one of the reasons anyway. Another is string interpolation. And I think there are more, though I can't remember them off the top of my head :-)

perl-diddler:

From the docs:

The "=>" operator is a synonym for the comma except that it causes its left operand to be interpreted as a string if it begins with a letter or underscore and is composed only of letters, digits and underscores. This includes operands that might otherwise be interpreted as operators, constants, single number v-strings or function calls. If in doubt about this behaviour, the left operand can be quoted explicitly.

So I'd suggest just using a comma:

my %map = (
  Page, 'mypage',
  Index, 'myindex',
);
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Notes: (1) Untested, as I don't recall ever using the constant module before, and (2) Yes, it looks strange to me, too!

...roboticus

Since constant works by installing subroutines, you can also write

Page() => 'mypage',
Index() => 'myindex',
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Swith the '=>' for ',' in your hash initialisation:

my %map = (
  Page, 'mypage',
  Index, 'myindex',
);
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People referred to it, but didn't explicitly state ... The way use constant works is it creates a subroutine which takes no arguments for your symbolic name, for example 'Page'. It's exactly the same as if you had:

sub Page() { return 1; }
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That works as far as preventing the value of Page from being altered, but it doesn't interpolate anywhere, because interpolation happens with variables, not barewords. As already mentioned, that's why Readonly has become the preferred method:

use Readonly;
Readonly my $Page => 1;
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That will interpolate the value wherever needed.

By the way, good style generally calls for ALL CAPS for global variables and constants. The reason is it stands out as a sin you've committed, motivating programmers to minimize the use to things that actually need it.