$ perl -le'
print defined $FH ? "defined" : "not defined";       
print fileno $FH ? "open" : "not open";              
open my $FH, "<", "test.txt" or die "test.txt: $!";
print defined $FH ? "defined" : "not defined";
print fileno $FH ? "open" : "not open";        
close $FH;                                     
print defined $FH ? "defined" : "not defined"; 
print fileno $FH ? "open" : "not open";
'
not defined
not open
defined
open
defined
not open