#! perl -sw
use strict;

## Open for reading and writing
open IO, '+<', 'junk.dat' or die $!;

seek IO, 11, 0;
print IO 'this overwrites';

close IO;

__END__

C:\test>type junk.dat
xxxxxxxxx
xxxxxxxxx
xxxxxxxxx
xxxxxxxxx

C:\test>junk59

C:\test>type junk.dat
xxxxxxxxx
this overwritesxxxxx
xxxxxxxxx