$input = <STDIN> if defined fileno(STDIN);

##</code><code>##

ken@titan ~/tmp
$ perl -E 'my $input = "none"; $input = <STDIN> if defined fileno(STDIN); say $input'
qwerty
qwerty


ken@titan ~/tmp
$ perl -E 'my $input = "none"; close STDIN; $input = <STDIN> if defined fileno(STDIN); say $input'
none

ken@titan ~/tmp

##</code><code>##

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


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

C:\Users\ken\tmp>

##</code><code>##

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>

##</code><code>##

This is perl 5, version 30, subversion 0 (v5.30.0) built for cygwin-thread-multi
This is perl 5, version 30, subversion 0 (v5.30.0) built for MSWin32-x64-multi-thread