Perl - Replacing a record at a specific position

Hi,

Using Perl, how could you replace a record at a specific position of a fixed length file?    Also, need to do multiple replacement at different positions for the same record.

For example, the file contains the following record:

-->  Need to change THIS first then THIS second ...<rest of line, 700 chars total>

Note:  1st "THIS" is at positions 16-19 and 2nd "THIS" is at position 32-35.

Goal:

Replace position 16 to 19 (len = 4), "THIS", with "XXXX"
and
Replace position 32 to 35 (len = 4), "THIS", with "YYY" << len different

Results:

--> Need to change XXXX first then YYYS second ...<rest of line, 700 chars total>

Note:  The 2nd replacement replaced "THIS" with "YYYS"; 'S' was kept and the records length was not changed, contained 700 chars (same as original)

Thank you



 
I need to change several
I need to modify a fixed length file, at several positions o