I'm unable to duplicate your problem, running DB_File v1.75 on Debian stable:

#!/usr/bin/perl -w

use strict;
use DB_File;
use Fcntl;

unlink 'data.db';

tie my %hash, 'DB_File', 'data.db', O_RDWR|O_CREAT;

foreach my $size (100, 200, 500, 900, 1000) {
    my $k = 'k' x $size;
    my $v = 'v' x $size;
    $hash{ $k } = $v;
}

untie %hash;
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#!/usr/bin/perl -w

use strict;
use DB_File;
use Fcntl;

print "using version: $DB_File::VERSION\n";

tie my %hash, 'DB_File', 'data.db';

foreach my $k (sort keys %hash) {
    printf "%d => %d\n", length($k), length($hash{$k});
}
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