It doesn't work. The diamond operator opens the file and reads from it at the same time, you can't first open the file and then set the variable. Check the difference yourself: 
perl -pwe '$^I = ".bak"; s/[<>]/|/g' file1
perl -pwe 'BEGIN { $^I = ".bak" } s/[<>]/|/g' file2

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In reply to Re^7: Delete a line from files by choroba
in thread Delete a line from files by alokranjan

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