When a method which cannot be resolved is invoked on a file handle, perl loads modules from the IO namespace to resolve it. The notation autoflush STDOUT 1 is equivalent to STDOUT->autoflush(1), which triggers the loading of IO::Handle:

qwurx [shmem] ~> perl -e 'use strict; use warnings; print$_.$/ for sor
+t keys %INC; autoflush STDOUT 1'
strict.pm
warnings.pm
qwurx [shmem] ~> perl -e 'use strict; use warnings; STDOUT->autoflush(
+1); print$_.$/ for sort keys %INC'
Carp.pm
Exporter.pm
Fcntl.pm
IO.pm
IO/File.pm
IO/Handle.pm
IO/Seekable.pm
SelectSaver.pm
Symbol.pm
XSLoader.pm
strict.pm
warnings.pm
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This is documented in Method Call Variations in perlobj.