The Cookbook has a nice overview of the different DBM implementations and their features (p90). Here's the table

Feature	NDBM	SDBM	GDBM	DB
Linkage comes with Perl	yes	yes	yes	yes
Source bundled with Perl	no	yes	no	no
Source redistributable	no	yes	gpl	yes
FTPable	no	yes	yes	yes
Easy to build	N/A	yes	yes	ok
Often comes with Unix	yes	no	no	no
Builds ok on Unix	N/A	yes	yes	yes
Builds ok on Windows	N/A	yes	yes	yes
Code size	depends	small	big	big
Disk Usage	depends	small	big	ok
Speed	depends	slow	ok	fast
Block Size Limits	4K	1K	none	none
Byte order independent	no	no	no	yes
User-defined sort order	no	no	no	yes
Partial key lookups	no	no	no	yes

There are other caveats and info so check out chapter 14 of the Cookbook.

-derby