I'm not sure I understand what should be the key and what should be the value, so tweak this to your needs:

#! /usr/bin/perl
use warnings;
use strict;

my %source;
my $host;
while (<>) {
    if (/^ok: \[(.*)\] => \{/) {
        $host = $1;

    } elsif (/^\s+"(\S+)\s+nfs\s/) {
        push @{ $source{$1} }, $host;
        # Or maybe
        # push @{ $source{$host} }, $1;
    }
}

use Data::Dumper;
print Dumper \%source;
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