Assuming it's OK for severity/modified to be blank, you can avoid the warning by changing that to

if (   $e->{severity} || '' =~ /High/
            && $e->{modified} || '' =~ /($today|$yesterday)/ )
        {
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or

if (   defined $e->{severity}
            && defined $e->{modified}
            && $e->{severity} =~ /High/
            && $e->{modified} =~ /($today|$yesterday)/ )
        {
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On the other hand, you might need to look at the source of your data and see about making a change there to ensure that severity and modified are set on all records if they're supposed to be required.

Either way, I would recommend making the behaviour consistent by adding use warnings right after use strict, so that they'll be on in both OSes, not by turning them off in Windows.