It's because in the second instance you're looping over a constant value in the for and the trying to assign that in my_method e.g 
sub readstuff { return while <> }
print readstuff for 'a constant value';

__output__

Modification of a read-only value attempted at - line 1.

[download]
Where the while really compiles as return while defined($_ = <>). As has been mentioned before in IlyaM's while(<>) { ... } considered harmful, be careful when implicitly using $_ in subroutines and methods.
HTH

_________
broquaint

In reply to Re: Method call to tied hash leads to file read error by broquaint
in thread Method call to tied hash leads to file read error by Cirollo

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