It's unclear whether or not the array can contain a valid line which consists of the empty string or the character '0'. either of these possibilities would break your code. This problem can be fixed with:

@{$mon_obj->{FILE_DATA}} 
  = grep { defined $_ } @{$mon_obj->{FILE_DATA}};
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"Perl makes the fun jobs fun
and the boring jobs bearable" - me

Yes, that's an obvious refinement, which the original loop did not contain,
presumably because '' and '0' are either not present or not valid
so I preserved that behavior in the grep condition.