How to reverse lines in a file using perl -e?

To reverse lines of input using a Perl one-liner, it’s important to understand how Perl command-line flags work and how to structure your code so it runs correctly in the shell and sandbox.

Understanding the Perl one-liner flags

• -n: wraps your code in a loop like while (<>) reading one line at a time but does not print automatically.
• END { ... }: a special block that runs after all input has been read, perfect for outputting accumulated data.
• The filename(s) must be provided as separate arguments outside the Perl code quotes, otherwise Perl parses incorrectly.

Why the original code fails in some sandboxes

• Omitting spaces between code and filename causes Perl to treat the filename as part of the code, leading to syntax errors.
• Using the wrong quoting or improper usage of flags results in bareword or operator errors.
• To run with perl - (reading code from STDIN), you must write the Perl code as a script block rather than a one-liner with input filenames.

Correct, runnable example (no file access)

Here is a complete Perl script snippet you can run with perl - that reads lines from standard input, stores them, then prints them reversed on exit.

use strict;
use warnings;

my @lines;
while ( <> ) {
    push @lines, $_;
}
print reverse @lines;

You can execute this as:

echo -e "line one\nline two\nline three" | perl -

Paste the code above into the standard input, then press Ctrl-D (Unix) or Ctrl-Z (Windows) to end input and see the reversed lines:

line three
line two
line one

Notes on Perl syntax and context

• $_ is the default variable that holds the current input line when reading <>.
• @lines is an array collecting all lines. Using push appends to it.
• reverse @lines returns the list in reversed order, which print outputs.
• This script uses standard I/O and does not require any files or modules, safe for sandbox execution.

Summary

• For one-liners reading from files, use: perl -ne 'push @lines, $_; END { print reverse @lines }' filename
• When using perl - (code from STDIN) with no files, wrap your logic in a script reading from standard input by while(<>).
• Always ensure filenames are outside Perl code quotes, and be mindful of shell quoting and syntax.

Following these guidelines will let you reverse lines safely and portably in Perl.