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$hash{ $_ } = undef for 1 .. 1_000_000;

undef $foo{ $_ } for 1 .. 1_000_000;

use DB_File;

...
1069482523
1069482799  # 276 secs (4.6 mins)
 1000000

print time; 
$hash{ substr $_, 0, 4 } .= $/. $_ for 1 .. 1_000_000; $hash{$_} .= $/
+ for keys %hash; 
...
1069484612
1069484656 # 44 secs.
 1000000