The multiple functions are probably overkill for this, but in general this looks decent. (I'd use a while loop instead of a do { } unless, but that is partly taste.)

'$flag' isn't a global variable (conceptually): it's a flag variable who's scope includes the while loop it controls. The fact that in this case that means it is at the top-level scope isn't really relevant. The only real way to write this without the variable would be to have do_it be in the loop condition itself, and that's more ugly. (And less expandable.)

I'd be tempted to rename '$flag' to '$again': Then do_it returns whether we want to do it again. But that's just me reading it and having fun. ;)

Oh, and this:

    my $drink         = shift;
    my $positions_ref = shift;
    my $locations_ref = shift;
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Can be shortened to this:

    my ($drink, $positions_ref, $locations_ref) = @_;
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