Thank you for the help and information. And from now on I will expect me and others to close DATA on finishing with it.

Here is what happens when 2 separate packages leave their DATA opened. warn reports only the last one being opened. Is it because the interpreter will detect opened DATA and localise it?

package X;

while(<DATA>){ print "X:".$_ }
#close(DATA);

sub check_data { return defined fileno(DATA) }

1;
__DATA__
1
2
3
[download]

package Y;

while(<DATA>){ print "Y:".$_ }
#close(DATA);

sub check_data { return defined fileno(DATA) }

1;
__DATA__
10
20
30
40
50
60
[download]

# main 
use lib '.';
use strict;
use warnings;
use X;
use Y;

warn "hehh";

print "X opened: ".X::check_data()
 ."\n"."Y opened: ".Y::check_data()
 ."\n";
[download]

X:1
X:2
X:3
Y:10
Y:20
Y:30
Y:40
Y:50
Y:60
hehh at x.pm line 7, <DATA> line 6.
Y opened: 1
Y opened: 1
[download]