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Yes, this is a well known behavior/feature called autovivification. It basically means that Perl will create the variable on the fly. The use strict pragma will only stop autovivification of variables, but not hash elements. To test for the existance of a hash key with out creating it, you must use the exists function. Use along the lines of if exists $hash->{1} instead.

perldoc -f exists

exists EXPR
             Given an expression that specifies a hash element or
             array element, returns true if the specified element
             in the hash or array has ever been initialized, even
             if the corresponding value is undefined.  The
             element is not autovivified if it doesn't exist.

                 print "Exists\n"    if exists $hash{$key};
                 print "Defined\n"   if defined $hash{$key};
                 print "True\n"      if $hash{$key};

                 print "Exists\n"    if exists $array[$index];
                 print "Defined\n"   if defined $array[$index];
                 print "True\n"      if $array[$index];

             A hash or array element can be true only if it's
             defined, and defined if it exists, but the reverse
             doesn't necessarily hold true.

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In reply to Re: Why does testing a key create a hash? by Roger
in thread Why does testing a key create a hash? by johnnywang

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