According to the Perl Survey, performed over a year ago now predating 5.10, as their minimum version of Perl people are using...
4.x 3%
5.0.x 3%
5.4.x 2%
5.5.x 6%
5.6.x 17%
5.8.x 66%
and the maximum is...
5.6.x 3%
5.8.x 88%
5.9.x 5%
5.6.1 and 5.8.8 are most popular within their major group. The missing percentages are miscellaneous versions.
Since the survey didn't define what "using" means, ie. if you're actually running production code on it or if you just test your software against it for backwards compatibility, or you just have it around for kicks, the minimum can be taken with a grain of salt.
This data, and the general lack of feedback from users, lead me to drop support for 5.5.x from the module installation toolchain modules I maintain (MakeMaker and Test::More) effectively end-of-lifing 5.5.x. 5.6 has had a stay of execution, but is still on death row.
I personally only recently switched from 5.8.8 to 5.10.0. I know of places still using 5.6.1 in legacy apps, but they've moved to 5.8.8 for as much as possible and all new dev. I don't know of anyone still using 5.5 in production. For backwards compatibility I test against 5.6.2, 5.8.8 and 5.10.0. The numbers say I should be testing 5.6.1 instead of 5.6.2 but there's only so far I can be arsed by folks who won't even upgrade to the latest bugfix release in their line.