thanks for the reply moritz but i'm assigning the value of $word64 based on a condition.Here is the code:

my $hw = `uname -i`;
my $word64 = 1 if ( $hw =~/x86_64|ia32e/);
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Now how can i use constant for this one?

Force that stuff to happen at compile time using BEGIN:

my $word64;
BEGIN {
  my $hw = `uname -i`;
  $word64 = 1 if $hw =~/x86_64|ia32e/;
}
use constant CRACKLIB => ($word64 ? "/usr/lib64/cracklib_dict" : "/usr
+/lib/cracklib_dict");
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perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

I don't see how that works. The code still runs at run-time, and the use statement still runs at compile time. Using  $x instead of  $x == 1 in the ternary eliminates a "Use of uninitialized value $x in numeric eq (==) ..." warning, but that's all.

>perl -wMstrict -le
"my $x;
 BEGIN {
   my $m = `uname -m`;
   print qq{m '$m'} if $m =~ /x86/;
   $x = 1 if $m =~ /x86/;
   use constant FOO => ($x ? 'true' : 'false');
   }
 print FOO;
 print qq{x '$x'};
"
m 'x86
'
false
x '1'
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Update: This seems to work:

>perl -wMstrict -le
"BEGIN {
   use constant X   => (`uname -m` =~ /x86/) ? 1 : 0;
   use constant FOO => (X ? 'true' : 'false');
   }
 print FOO;
 print q{X '}, X, q{'};
"
true
X '1'
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Update: Oh, wait... I didn't catch the fact that the  use constant statement is outside the BEGIN block! tobyink's code works as advertised (per tobyink's reply which I didn't see until after I had posted this update). Evidently it helps if you test the actual code. Nevermind.

>perl -wMstrict -le
"my $x;
 BEGIN {
   my $m = `uname -m`;
   print qq{m '$m'} if $m =~ /x86/;
   $x = 1 if $m =~ /x86/;
   }
 use constant FOO => ($x ? 'true' : 'false');
 print FOO;
 print qq{x '$x'};
"
m 'x86
'
true
x '1'
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thanks tobyink, its working fine now.