You could do this at the prompt with something like

perl -ne 'print if / (\S) \1 \1$/' data.txt
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assuming your file is named data.txt. If you want to save that to a second file then

perl -ne 'print if / (\S) \1 \1$/' data.txt > second.file
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The -ne flags are very handy at the command line. The -e flag means "evaluate this code". The -n flag doesn't have a convenient mnemonic but wraps a loop around the code for every line of input.

For example...

perl -e 'print "hello world\n"'
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... is a nice canonical example. And...

perl -ne 'print "hello world. Look at this: $_"' data.txt
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...will prepend text to every line of data.txt and print the result.

more docs for command line options at perlrun

more docs for regular expressions at perlre