The sysseek documentation says:

Returns the new position, or the undefined value on failure. A position of zero is returned as the string "0 but true"; thus sysseek returns true on success and false on failure, yet you can still easily determine the new position.

my $rv = sysseek(...);

# Handle error
die(...)
   if not $rv;

# Convert "0 but true" to 0
$rv += 0;

...
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Update: Oops, initial code sample was for sysread. Fixed for sysseek.

Update: ug, me need sleep. Applied tye's fix. Now going to bed.

"0 but true" is, of course, true, so $rv ||= 0; does no good. (:

$rv= 0   if  0 == $rv;
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Or, tersely

$rv += 0;
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- tye