File I/O

Code

use strict;
use warnings;

# Write
open(my $fh, '>', 'output.txt') or die "Cannot open: $!";
print $fh "Line 1\n";
print $fh "Line 2\n";
close($fh);

# Read line by line
open(my $in, '<', 'input.txt') or die "Cannot open: $!";
while (my $line = <$in>) {
  chomp $line;  # remove newline
  print "Read: $line\n";
}
close($in);

# Slurp entire file
open(my $slurp, '<', 'data.txt') or die $!;
my $content = do { local $/; <$slurp> };
close($slurp);

# Append
open(my $log, '>>', 'app.log') or die $!;
print $log "[" . localtime() . "] event\n";
close($log);

# Read all lines into array
open(my $arr, '<', 'data.txt') or die $!;
my @lines = <$arr>;
close($arr);

# In-place edit (perl -i -pe 's/old/new/g' file.txt)
{
  local $^I = '.bak';  # backup extension
  local @ARGV = ('data.txt');
  while (<>) {
    s/old/new/g;
    print;
  }
}

Explanation

Always use the 3-arg open and check for errors (or die $!). Lexical filehandles (my $fh) auto-close on scope exit. local $/ temporarily disables the input record separator, slurping the whole file. chomp removes the trailing newline. The diamond operator <> reads from files in @ARGV or STDIN. $^I enables in-place editing (used by -i command-line flag).

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