$ perl -e'flush STDOUT;'

$ perl -e'no feature qw( indirect ); flush STDOUT;'
Bareword found where operator expected (Do you need to predeclare "flush"?) at -e line 1, near "flush STDOUT"
syntax error at -e line 1, near "flush STDOUT"
Execution of -e aborted due to compilation errors.

$ perl -e'use v5.36; flush STDOUT;'
Bareword found where operator expected (Do you need to predeclare "flush"?) at -e line 1, near "flush STDOUT"
syntax error at -e line 1, near "flush STDOUT"
Execution of -e aborted due to compilation errors.