You should use strict and warnings. Particularly

print ( $#keys + 1 ) . "\n";
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is not what you want.

Concerning you question, you're certainly hitting data pages, as you retrieving data values from database with c_get. Sorry, have no idea how to avoid this.

PS it looks like the following is slightly faster:

my $db  = tie my %hash, 'BerkeleyDB::Btree',
    -Filename => 'data.dbm';
my @keys = keys %hash;
print 0+@keys, "\n";
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