Two versions:

sub hexout
{
  #23456789012345678901234567890123 - 33 char
  $_=unpack"H*",pop;s/(..)/$1 /g;$_
}
[download]

but it appends a space at the end

sub hexout
{
  #234567890123456789012 - 22
  sprintf'%*v2X',' ',pop
}
[download]

#2 was with a little help from the sprintf docs

Update: Spent a little too much time. MrNobo1024 beat me to the punch ;-)