This is UNTESTED but you may want code something like this:

Update to while loop conditional.

use constant HEADER_LEN => 32;
use constant RECORD_LEN => 286;
use constant DUMMY      => "\0" x RECORD_LEN;

if ( $fname ) {

    open my $OUTPUT, '>:raw', "$fname.tmp" or die "Cannot open '$fname
+.tmp' $!";
    open my $INPUT,  '<:raw', $fname       or die "Cannot open '$fname
+' $!";

RECORD:
    {
        my ( $offset, $bytes_read, $header ) = ( 0, 0, '' );
#        while ( $bytes_read < HEADER_LEN ) {
        while ( $offset < HEADER_LEN ) {
            defined( $bytes_read = read $INPUT, $header, HEADER_LEN - 
+$offset, $offset )
                or die "Cannot read from '$fname' $!";
            $bytes_read or last RECORD;  # EOF
            $offset += $bytes_read;
            }

        # format may be either 'x30 n' or 'x30 S' depending on endiane
+ss?
        my $total_record = unpack 'x30 n', $header;

        my $records_len = $total_record * RECORD_LEN;

        ( $offset, $bytes_read, my $data ) = ( 0, 0, '' );
#        while ( $bytes_read < $records_len ) {
        while ( $offset < $records_len ) {
            defined( $bytes_read = read $INPUT, $data, $records_len - 
+$offset, $offset )
                or die "Cannot read from '$fname' $!";
            $bytes_read or last RECORD;  # EOF
            $offset += $bytes_read;
            }

        $header = pack 'a30 n', $header, 3;

        print $OUTPUT $header, $data, $total_record == 2 ? DUMMY : '';
        redo;
        }

    }
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