my %severity = ( critical => -1, OK => 1 );
my @sorted = sort {
    $severity{ $a->{severity} } <=> $severity{ $b->{severity} }
    ||
    $severity{ $a->{severity} } * ( $a->{event_age} <=> $b->{event_age
+} )
} @records;
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i.e. when comparing two elements, first compare their severity, and then compare their event age, but reverse the result for the critical ones.

Update: simplified by using negative severity for the critical.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]