Perhaps for VMS and suchlike, I don't know. On AS build 630 (perl 5.6.1)/Windows95, which does not support flock, my code gives:

C:\download>perl -c flock.pl
flock.pl syntax OK
C:\download>perl flock.pl
flock not supported! flock() unimplemented at flock.pl line
[download]

at which point winders wrote off the edge of the screen and lost the line number ;-)

My 'fatal error' jargon came directly from 'perldoc -f flock'.

After Compline,
Zaxo