Though there's more online documentation than you can comfortably read, it's still nice to have a paper copy of Programming Perl, and perhaps also Learning Perl for a gentler introduction.

Additionally, as a bioinformatics programmer, you will want to get very comfortable working with Perl's data structures; hashes are one of Perl's huge strengths, and you'll need proficiency in manipulating hashes of hashes, arrays of hashes, arrays of arrays of hashes of arrays, etc. This is covered online in the perllol docs.