It's probably overkill, but I'll give it a plug, anyway:

C:\>perl -MMath::MPFR -le "print Math::MPFR->new('2.5', 8);"
2.625
[download]

Math::GMPf is another perl module that can convert octal floating point values to decimal.
Are there any other modules doing that ? (I would expect so, but I haven't checked.)
I couldn't spot any built-in perl functions that will readily do this.

Cheers,
Rob