use 5.010;
my @chunks = split " ", "Debashis Pranab roy Nagendra roy murali mohan
+ roy";
for (1..$#chunks) {
    say $chunks[$_-1] if $chunks[$_] eq "roy";
}
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Update: change range to start at 1, so that a leading "roy" doesn't result in the last item being printed, as pointed out by jethro++.