In that case you will need to alter the inner hashrefs, not the outer array, viz:

#!/usr/bin/env perl 
use strict;
use warnings;
use Data::Dumper;

my @eventrecords = (
    { eventid => 2667 },
    { eventid => 2666 },
    { eventid => 2656 }
);

for my $i (0 .. $#eventrecords) {
    $eventrecords[$i]->{line} = $i + 1;
}

print Dumper \@eventrecords;
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