BER uses a Tag Length Value (TLV) format where as PER uses a compact form of encoding a structure (aligned (Aligned to octets by the use of padding bits ) or unaligned(no need of alignment as exact size of data is sent so its more compact ) format.
TLV - BER
In case of TLV format each member will be having a tag number which uniquely identifies a member, length specifies its size and value contents the value of the field according to length specified.
PER format each member converted to an continuous buffer by taking its member identifier and value.
For introductary learning u can go through this tutorial - http://www.oss.com/asn1/resources/asn1-made-simple/introduction.html
For hands on to make a simple asn file taking and encode it with different encoding rules u will learn a lot on it.
Its really a good website and u can use his opensource compilers too for learning in to deep, it supports all encoding decoding format but not aligned per format.