If the snippets in the OP are what you're actually running, then I assume that either you did not 'use strict', or else you have a variable declared elsewhere called '$guid' and you are using it where you shouldn't be. Here's the problem in your first while loop:

    my ($id, $accno) = split /\|/, $_;
    push @{$hashList{$guid}}, $accno;
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I think you want:   push @{$hashList{id}}, $accno;

As it is, all records in the first input are being pushed onto a single array. Then, in the second while loop, it may be you are running into that one string that matches the one hash key that contains the one big array. And this statement:

        for my $accno (@{$hashList{$_}})
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tries to make a copy of the array. Boom. (I can't be sure this is what's happening based on the small amount of info you gave, but it looks plausible.)

UPDATE: Actually, I'm not sure that the for-loop expression there is copying the array -- seems like it shouldn't have to. Still, you need to fix how the hash gets loaded.