It will NOT work. I can't think Why?

Because the MIME-encoded parts are not properly terminated. GIGO. They should end with ?=. By changing them to ensure that they are properly terminated they work fine.

#!/usr/bin/env perl 
use strict;
use warnings;
use Encode qw(encode decode);

my @subject = ("Room Rush \303\242\302\200\302\223 Enjoy 25% off on yo
+ur stay. (raw: Room Rush =?utf-8?b?4oCT?= Enjoy 25% off on your stay.
+)",
"Last Day To Enjoy Extra 15% OFF On Everything For NDB Credit Cards (r
+aw: =?utf-8?Q?Last=20Day=20To=20Enjoy=20Extra=2015%=20OFF=20On=20Ever
+ything=20For=20NDB=20Credit=20Cards?=)",
"Sing Along and Dance with Desmond De Silva at Pegasus Reef Hotel! (ra
+w: =?utf-8?Q?Sing=20Along=20and=20Dance=20with=20Desmond=20De=20Silva
+=20at=20Pegasus=20Reef=20Hotel=21?=)");

for my $subj (@subject) {
    my $decoded = decode ("MIME-Header", $subj);
    print encode ("UTF-8", $decoded) . "\n";
}
[download]

2 days ago, I got an email with below subject

my $subject = "=?GB18030?B?XXXXXXXXXX?=";
[download]

then, When I ran the code, I got below error

Unknown encoding "GB18030" at /usr/lib64/perl5/Encode.pm line 174

I searched it. Then I came to know that GB 18030 is a Chinese government standard.

I found below 2 Urls

https://perldoc.perl.org/Encode/CN.html

https://stackoverflow.com/questions/6105316/how-to-convert-from-gbk-encoding-to-utf-8-encoding-in-perl

they talk about Encode::HanExtra

So, Added below line to my code. then, It started working.

use Encode::HanExtra;
[download]

So, Here's my UPDATED code. I think it's worth sharing...

#!/usr/bin/perl
use strict;
use warnings;
use Encode qw(encode decode);
use Encode::HanExtra;
no warnings 'utf8'; 

my @subject = ("Room Rush \303\242\302\200\302\223 Enjoy 25% off on yo
+ur stay. (raw: Room Rush =?utf-8?b?4oCT?= Enjoy 25% off on your stay.
+)",
"Last Day To Enjoy Extra 15% OFF On Everything For NDB Credit Cards (r
+aw: =?utf-8?Q?Last=20Day=20To=20Enjoy=20Extra=2015%=20OFF=20On=20Ever
+ything=20For=20NDB=20Credit=20Cards?=)",
"Sing Along and Dance with Desmond De Silva at Pegasus Reef Hotel! (ra
+w: =?utf-8?Q?Sing=20Along=20and=20Dance=20with=20Desmond=20De=20Silva
+=20at=20Pegasus=20Reef=20Hotel=21?=)",
"RE: Weekly Report (CommercialLegal) -\303\202\302\240 2 July 2018 to 
+5 July 2018 (raw: =?UTF-8?Q?RE:_Weekly_Report_=28CommercialLeg?=\t=?U
+TF-8?Q?al=29_-=C2=A0_2_July_2018_to_5_July_201?=\t=)",
"How are you",
"=?GB18030?B?XXXXXXXXXX?=");

for my $subj (@subject) {
    my $subject_decoded = decode("MIME-Header", $subj);
    print $subject_decoded;
    print "\n";
}
[download]

Have a nice day to all Perl Monks

Thanks for enlightening me. Since I was busy, I had to take much time to reply you. Sorry for it. Thanks everyone for their wonderful efforts.