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use strict;
use warnings;
...
print '$hash{a} ',     ( $hash{a}            ) ? "true"    : "not true
+";
print '$hash{a} ',     ( defined $hash{a}    ) ? "defined" : "not defi
+ned";
print '$hash{a} ',     ( exists $hash{a}     ) ? "exists"  : "doesn't 
+exist";

$hash{a} not true
$hash{a} not defined
...
$hash{a} true
$hash{a} defined
$hash{a} exists