1. Is this using too many paren statements, or is this just per coder's discretion?

2. This regex matches on 'C3550XL' and I'm not sure why

_________
broquaint

You parsed it incorrectly. (I parsed broquaint's regex incorrectly). By just adding whitespace I get the following:

/ C (?: 800 ) |
  (?: (?: 35 |
          29 )
      (?: (?: 50 ) |
          (?: 00XL )
      )
  ) /x
[download]

Which shows that some (?:) were completely useless. This imposes zero runtime overhead so it only really matters because it makes it harder for the next programmer to read. In removing the useless (?:) groups I get.

/ C800 |
   (?: 35 | 29 )
   (?: 50 | 00XL )
   /x
[download]

If your strings are stand-alone, then using anchors (\A & \Z (or ^ & $ unless your using the /m option)) will work fine, but if your trying to detect matches for this regex within the context of a larger string, then you can acheive this using a few zero-width assertions.

#! perl -slw
use strict;

while( <DATA> ) {
    chomp;
    print if m[
        \bC                     # Starting with C preceeded by a non-\
+w char
        (?:                     # and either
            800                 # 800
            |                   # or
            (?: 29 | 35 )       # 29 or 35
            (?:                 # and
                (?: 50(?!XL) )  # 50 not followed by XL
                |               # or
                (?: 00XL )      # 00XL
            )
        )\b                     # followed by a word break
    ]x;
}
__DATA__
This  is the spec for the ABC800. This device is a
This  is the spec for the C800. This device is a
This  is the spec for the C2900. This device is a
This  is the spec for the C2950. This device is a
This  is the spec for the C2900XL. This device is a
This  is the spec for the C2950XL. This device is a
This  is the spec for the C3500. This device is a
This  is the spec for the C3550. This device is a
This  is the spec for the C3500XL. This device is a
This  is the spec for the C3500XLM. This device is a
This  is the spec for the C3550XL. This device is a

__OUTPUT__
D:\Perl\test>junk
This  is the spec for the C800. This device is a
This  is the spec for the C2950. This device is a
This  is the spec for the C2900XL. This device is a
This  is the spec for the C3550. This device is a
This  is the spec for the C3500XL. This device is a
[download]