This bit is odd, for a file that was opened for write:

          seek (OUT,2,1);
          read (OUT, my $num,4);
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Why don't you just use a regex to pick out the segment markers?

     if (/^(.*?)$delim\s\s(....)(.*)$/) {
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Now the last bit of your last file is in $1, and the four characters to make into a filename are in $2. The start of your next file is in $3.

Look into the tr/// operator for deleting control characters.