Question

I would like to understand this expression meaning.

$req_msg =~ s/ \${$toReplace}/$replacements->{$toReplace}/g;

Answer 1

$req_msg =~ s/ \${$toReplace}/$replacements->{$toReplace}/g;

s is used for substitution. $content=~ s/old_value/new_value/modifier; (modifier can be i, g, x, along or combination)

Ex:

$content = "Hi I am a coder and I like coding very much!";
$content =~ s/i/eye/i;

now $content will contain “Heye eye am a coders and eye like coding very much”

In the same way ${$toReplace} which simply means a scalar reference is the old value which needs to be replace and $replacements->{$toReplace} means $replacements is a hash reference whose key is $toReplace .
It is smiliar to $hash_value = hash_ref->{key};

whereever it finds the value returned by scalar reference , gets replace by hash reference’s key with the corresponding value found in $req_msg

But I guess you asked this question because you got blank replacement. That may be due to scalar reference problem.

This code snippet may help in removing your doubt.

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

my $value = "Jassi";
my $scalar_ref = \$value;

print "scalar_ref = $scalar_ref \n and value = $value and ${$scalar_ref}\n";
my %hash = ("Jassi", "aliencoders");
my $hash_ref = \%hash;
my $reg_msg = "Hi this is Jassi";
print "reg_msg = $reg_msg \n";
$reg_msg =~ s/${$scalar_ref}/$hash_ref->{${$scalar_ref}}/;
print "reg_msg after s = $reg_msg\n";

See the second last line!