PS C:\Users\ken\tmp> perl -E 'my $input = "none"; close STDIN; eval { defined fileno(STDIN) } and do { $input = <STDIN> }; say $input'
none
PS C:\Users\ken\tmp> perl -E 'my $input = "none"; $input = <STDIN> if defined fileno(STDIN); say $input'
qwerty
qwerty

PS C:\Users\ken\tmp> perl -E 'my $input = "none"; close STDIN; $input = <STDIN> if defined fileno(STDIN); say $input'
none
PS C:\Users\ken\tmp>