You might try using the -t file test operator, this should be able to tell you whether a handle is opened to TTY (and hence not flock()able) and not to a file:

print "Yow!" if -t STDOUT;
open SAVOUT, '>&STDOUT';
close STDOUT;
open STDOUT, '>stdout.txt';
print "Yow!" if -t STDOUT;
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/J\