gistfile1.pl

#!/usr/bin/env perl
use strict;
use warnings;
use 5.010;

my $string = "This is the real string.";

system 'echo ' . "'" . '$string' . "'" . '> example.txt';
# the command is: 
# % echo '$string' > example.txt
# so the file literally contains the line: $string

open(my $in,  "<",  "example.txt");

my $read = <$in>;

# perl reads the file literally, i.e. as if saying:
# my $first_line = '$string';
# my $read = $first_line;
# perl doesn't know that $string is a variable in the file

say "\$string = '$string'";
say "\$read = '$read'";

# but when you evaluate it, the variable is "discovered"
my $ev = eval $read;
say "\$ev = '$ev'";