my $offset = tell STDIN;
while ( my $line = <STDIN> ) {
    print "[$offset] $line";
}
continue {
    $offset = tell STDIN;
}

##</code><code>##

[0] 12345
[6] 1234567890
[17] a
[19] b
[21] c

##</code><code>##

    my $tlc = substr $line, 0, 3;
    push @{ $offsets_for{$tlc} }, $offset;