SPOILER! -- you asked for it.

# I interpolated the numerical constants built from $[, $|, $m, 
# which have the values 0, 1, 4 resp.
# By indenting the code and changing ?:, map, etc to proper 
# loops and ifs, and also doing some other minor changes, we get this.

$|= 1;
$m= -1;
while ($line=<DATA>) {
  if ( $line=~m;\S;s )
    { ++$m }
  else {
    # $m is always 4 at this point
    for $n (@s, -64) { # loop A
      print chr ( $n +96 -($n!=16 ? 0 : 32) )
      }
    @s= ();
    $m= -1;
    }
  $g= 0;
  while ($line=~m/(....)/g) { # loop B
    $1=~m/;/ and $s[$g]+= 1<<$m;
    $g++;
    }
  }

# The code reads the DATA section in, and splits every line 
# to four-char long parts.  Nothing counts, only whether 
# those parts have a semicolon or not. I've actually changed all 
# other glyphs to #'s, as `'.,: is all the same for the script.
#
# The obfu processes each paragraph at once. For each line 
# of the paragraph, loop B runs once.  While it runs, $m is 
# the line number in the paragraph minus one.  As you can see, 
# the loop sets bit $m of $s[$g] iff the $m+1'th line of the 
# $g'th big letter has a semicolon.
#
# Then, at the end of each paragraph (ie line of big letters), 
# loop A runs.  At then, the data from each big letter is stored 
# in @s, one letter each, so the loop is looping in the big letters.  
# And wow, it turns out, that the code just prints chr(96+$n), the 
# $n'th letter of the alphabet. (subtracting 32 for P to make it a 
# capital).  At the end, it prints a space too, which it gets from 
# the number -64 (really encoded as -($m<<$m-$|)-($|<<$m+$|) ).
#
# So, in fact, each big letter in the DATA section just 
# encodes itself, the code is in fact doing some kind of OCR.
#
# Example of the encoding:
#
#  E      R
# ;## 1  ###
# #      # ; 2
# ;#  4  ###
# #      # #
# ###    ; # 16 +
# ----------------
#     5      18
#
# And indeed, E is the 5th letter of the alphabet, 
# R is the 16th.
#

__DATA__
 ### # ; #;# ###
  ;  # # ;    # 
  #  # ; ###  ; 
  ;  # #   #  # 
 ##  #;# #;#  ; 

 #;# ### #;# ### # # ;#; ###
 # # ; ; ; ;  #  # # #   # ;
 ### # ; # ;  ;  ### #;  ###
 # # ; ; # ;  #  # ; #   # #
 # # # # ###  ;  # # ### # ;

 ### ##; ### #  
 # # #   ; # #  
 ##  #;  ### ;  
 #   #   # # ;  
 ;   ### ; ; ###

 # # #;# #;# ; # ;## ###
 # # # # ;   # ; #   # ;
 ### ### #   ### ;#  ###
 ; # # # #   ; # #   # #
 # # # # ### # # ### ; #

__END__
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Update: Corrected DATA section.