Under the assumption that you do not want rows with undefined values in your output (however they got there):

my @matrix_filtered = grep { defined( $_->[0] ) && $_->[0] =~ /$to_keep/ } @matrix;

You could simply silence the warning, but then you will get the bogus rows in your output if '' =~ /$to_keep/ is true.

In reply to Re: "Use of uninitialized value in pattern match (m//)" warning, doing a grep on 2D array by Anonymous Monk
in thread "Use of uninitialized value in pattern match (m//)" warning, doing a grep on 2D array by soblanc

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