actually this sounds more like a task for substr than a regexp. Here's a tested fragment

my $bit_string = "01111110000000000000010001011100100101111110";

# starting at the front, take 8 characters
my $flag = substr($bit_string,0,8); 

# start 8 chars in, get 16 out.
my $byte_count = substr($bit_string,8,16); 

# start at the END and get 8 characters;
my $flag2 = substr($bit_string,-8,8); 

# the mystery part. 24 chars in, length of parts we haven't taken out 
+yet.
my $the_rest = substr($bit_string,24,(length($bit_string) - 24 - 8));
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HTH

Update The things you find on CPAN. NetPacket may do exactly what you need

/\/\averick
perl -l -e "eval pack('h*','072796e6470272f2c5f2c5166756279636b672');"