{
    # Set the record separator to match the data.
    # In this example I have a blank line between
    # the lines I want to match, so set $/ to two
    # newlines.
    # Note that I'm setting $/ in its own data block (the empty
    # brackets) to localize the record separator change.
    $/ = "\n\n";

    while (<DATA>) {
        if(/First line of text[\n]second line of text[\n]in the third line I have (\d*\.\d*)/s) {
            print "Captured ($1)\n";
        }
    }
}

__DATA__
First line of text
second line of text
in the third line I have 3.14159

Here is another line of text
second line of text
third line has 1.41421356

First line of text
second line of text
in the third line I have 1.73205081