G'day demichi,

Welcome to the monastery.

First, some issues with your post:

• Second field:
  • Sample data shows no spaces: POLS and POLN (twice)
  • Attempted regex shows no matching of spaces for this field: /^(\w+)\s+(\w+)\s+.../
  • Description says: "1-n characters with 0-n spaces"
  I've assumed no spaces.
• Third record, fourth field:
  • Sample data shows: DemoSolutions
  • Actual and expected output show: Demo
  I've assumed a typo.
• Terminal spaces in sample data records:
  • First record has none
  • Second and third records have one
  • Attempted regex (/...\s+/) seems to indicate one or more (although, there's no end-of-line assertion)
  I've allowed for zero or more.
• Actual and expected output show a terminal semicolon for each record but this doesn't equate with a field separator.
  I've used ($) to generate a final, zero-length field; you may want to change this.

Taking those assumptions (and other points) into account, this regex achieves what I think you want:

#!/usr/bin/env perl -l

use strict;
use warnings;

my $re = qr{
    ^                                                     # start of l
+ine
    (\S+)\s+(\S+)\s+(\S+)\s+                              # 3 fields, 
+no spaces
    (.+?)\s+                                              # 1 field, +
+/- spaces
    (\S+)\s+(\S+)\s+(\S+)\s+(\S+)\s+(\S+)\s+(\S+)\s+(\S+) # 7 fields, 
+no spaces
    \s*                                                   # possible s
+paces
    ($)                                                   # end of lin
+e
}x;

print join ';' => /$re/ while <DATA>;

__DATA__
PID  POLS U(%) POOL_NAME    Seq# Num LDEV# H(%) VCAP(%) TYPE PM
003  POLN   0  Bad name with spaces   13453   2 61443  80   -  OPEN N 
002  POLN  52  DemoSolutions   54068  7 61454  80   -  OPEN N
[download]

Output:

PID;POLS;U(%);POOL_NAME;Seq#;Num;LDEV#;H(%);VCAP(%);TYPE;PM;
003;POLN;0;Bad name with spaces;13453;2;61443;80;-;OPEN;N;
002;POLN;52;DemoSolutions;54068;7;61454;80;-;OPEN;N;
[download]