There is nothing special about STDOUT, it's just another file handle. seek() does work on STDOUT if STDOUT is seekable, e.g. a plain file. But (pseudo) TTYs usually aren't seekable, at least not for writing, so seek() doesn't work when STDOUT is not redirected.

#!/usr/bin/perl -w
use strict;

seek(STDOUT,14,0);
print "Now I'm there\n";
seek(STDOUT,0,0);
print "Now I'm here\n";
[download]

Run without redirection (perl foo.pl), and you get:

Now I'm there
Now I'm here
[download]

The seek()s simply fail, and because the code does not check for errors, seek() seems to be a dummy for STDOUT.

Run again, redirecting to a file (perl foo.pl > foo.txt; cat foo.txt), and the file contains:

Now I'm here
Now I'm there
[download]

Now let's play safe and add some error checking:

#!/usr/bin/perl -w
use strict;

seek(STDOUT,14,0) or warn $!;
print "Now I'm there\n";
seek(STDOUT,0,0) or warn $!;
print "Now I'm here\n";
[download]

Run again, without redirection:

Illegal seek at foo.pl line 4.
Now I'm there
Illegal seek at foo.pl line 6.
Now I'm here
[download]

So, seek() is not a dummy, it simply fails, because the TTY does not allow seek() for write access.

Redirecting to a file, as above, works without any warnings:

Now I'm here
Now I'm there
[download]

Alexander