Replace text in a file

Let's say file.txt contains the text "Hello World". The following Perl script will open file.txt, and then replace the text "World" with "Earth".

# Load the content of the file into an array
open(FH, '<', '/path/to/file.txt') or die "cannot open file $! \n";
@array = <FH>;
close(FH);

# Overwrite file.txt with the content of array, while also removing and/or replacing lines
open(FH, '>', '/path/to/file.txt') or die "cannot open file $! \n";
foreach $line (@array) {
  print FH $line unless ($line m/stuff to remove/);
}
close(FH);

Read and write to the same file

In this example, "Hello World" is appended to the file.

my $file = "example.txt";

# Open the original file
open(IN, '<', "$file") or die "Cannot open $file $! \n";

# Create a tmp file
open(OUT, '>>', "${file}.tmp") or die "Cannot create $file.tmp $! \n";

# Append something to the top of the tmp file
print OUT "Hello Top\n";

# Copy everything in the original file to the tmp file
while (<IN>) {
  $_ =~ s/old/new/g; # Replace "old" with "new"
  print OUT $_;
}

# Append something to the bottom of the tmp file
print OUT "Hello Bottom\n";

# Close both files
close(IN);
close(OUT);

# Delete the original file
unlink($file);

# Rename the tmp file to have the original file name
rename("${file}.tmp", $file);