my $binary = pack 'F*', (3.141592) x 10;  # make binary vector of length 80 bytes
 my $string = '...blah...<file fiop="foo" length="' . length($binary) .'"/>' . $binary . '</file>...blah...';

 my ($fiop, $length, $data)
    = $string =~ m{<file                   # tag anchor
                   \s+ fiop="([^"]+)"      # (fiop)
                   \s+ length="([^"]+)"    # (length)
                   />                      # end: start file tag
                   ((??{ "\\C{$2}" }))     # self modifying regex for binary stuff
                   </file>                 # end: file tag
                  }sx;

 print "$fiop, $length (data comes below)\n";
 print join ',', unpack("F*", $data);      # extract binary data again
 
 (my $notags = $string) =~ s{<file.+</file>}{};
 print "\n$notags\n";