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As for the "\1", it stores the first match contained in "()" brackets. To modify your file directly use this: perl -i -0pe 's/Raining(.*\n.*TODAY)/Sunny\1/g;s/(TODAY.*\n.*)Reading/\1Hiking/g' file

man perlrun
-p causes Perl to assume the following loop around your program, which makes it iterate over filename arguments somewhat like sed:
LINE:
while (<>) {
... ...

Try: perl -0pe 's/Raining(.*\n.*TODAY)/Sunny\1/g;s/(TODAY.*\n.*)Reading/\1Hiking/g' file