...
 my $binary = pack 'F*', (3.141592) x 8001; # this will dump a 64K+ binary chunk
 my $string = '...blah...<file fiop="foo" length="' . length($binary) .'"/>' . $binary . '</file>...blah...';

 my ($fiop, $length, $data);
 if( $string =~ m{<file \s+ fiop="([^"]+)" \s+ length="([^"]+)" />}gx ) {
    ($fiop, $length) = ($1, $2);                    # extract tag properties as usual
    $data = substr $string, pos($string), $length   # extract data by direct string copy
 }
 
 print "$fiop, $length\n";
 print join ',', unpack("F*", $data);

 (my $notags = $string) =~ s{<file.+</file>}{};
 print "\n$notags\n";
 ...