open FILE, "<whatever.txt" or die "Oops $!\n";
my @content = <FILE>;
close FILE;
for my $i ( 0 .. $#content -1 ) {
    $content[$i] = $some_edit if $content[$i+1] eq $some_next_line_bit;
}

open FILE, ">whatever.txt" or die "Oops $!\n";
print FILE @content;
close FILE;