If you want to detect change from one line to another, indeed something like if $foo ne $previousfoo will do.

But detecting the first occurrence of a chain is quite another problem that can be solved by memorizing the number of occurences as value associated to the string:

perl -ne 'chomp; print "new $_\n" unless $a{$_}++'  nm_of_processed_fi
+le
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The loop may have to be explicit depending on the source of your data.

-- stefp -- check out TeXmacs wiki