The diagnostics error is due to the fact that the reading of a file could return 0 which would evaluate to false in a boolean context, whereas you want to be testing for definedness. The reason you can't read from <DATA> within the loop is that you've already slurped it in the foreach. So in this case a while loop would make a lot more sense e.g 
use strict;
use diagnostics;

my $line = (<DATA>);
print "line ouside loop = $line";

while(my $line = <DATA>) {

    print "line from foreach = $line";

    $line = (<DATA>) or print "error reading filehandle\n";
    $line ||= "null\n";
    print "line inside loop = $line";

}

__DATA__
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HTH

_________
broquaint

In reply to Re: Trying to read a filehandle inside a loop by broquaint
in thread Trying to read a filehandle inside a loop by roik

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