if $currentrecord =~ m/^field2(.*)\nfield3/mi {
    $field2data = $1;
}


##</code><code>##

Here's two examples of the data encountered:
record 1:
field1: data 1 monday
field2: data 2 monday
field3: data 3 monday

record 2:
field1: data 1 tuesday
field2: data 2 tuesday
        tuesday details line 1
        tuesday details line 2
field3: data 3 tuesday