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The only caveat to the use constant pragma is that constants cannot be directly used as keys to a hash table. For example:

badconst.pl
  1. #!/usr/bin/perl
  2.  
  3. use warnings;
  4. use strict;
  5.  
  6. use constant PI => (atan2(1,0) * 2);
  7.  
  8. my %htable =
  9.     (
  10. 	2.7182	=> "Euler's constant",
  11. 	PI	=> "Pi",
  12. 	1.6180  => "golden ratio",
  13.     );
  14.  
  15. print "keys = ( " .  join(', ', keys %htable) . ")";
  16. print "\n";

This will print "keys = ( 1.618, PI, 2.7182 )" because the word "PI" in the context of a hash key is always interpreted as the string, not the constant symbol.

goodconst.pl
  1. #!/usr/bin/perl
  2.  
  3. use warnings;
  4. use strict;
  5.  
  6. use constant PI => (atan2(1,0) * 2);
  7.  
  8. my $x = PI;
  9. my %htable =
  10.     (
  11. 	2.7182	=> "Euler's constant",
  12. 	$x	=> "Pi",
  13. 	1.6180  => "golden ratio",
  14.     );
  15.  
  16. print "keys = ( " .  join(', ', keys %htable) . ")";
  17. print "\n";

This will correctly print "keys = ( 3.14159, 1.618, 2.7182 )" but is not very elegant.


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