Now I see

The s in m//s means, that . also matches \n

 $cmdout =~ / \n ( .* procdump .*) \n /mx will work, because //m means that . does not match \n

use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
    qr/ \n ( .* procdump .*) \n /mx
)->explain;
__END__
The regular expression:

(?mx-is: \n ( .* procdump .*) \n )

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?mx-is:                 group, but do not capture (with ^ and $
                         matching start and end of line)
                         (disregarding whitespace and comments)
                         (case-sensitive) (with . not matching \n):
----------------------------------------------------------------------
  \n                       '\n' (newline)
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
    procdump                 'procdump'
----------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \n                       '\n' (newline)
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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You can also write $cmdout =~ / ^ ( .* procdump .*) $ /mx