The solution, as others have said, is to use chomp to strip the newline characters from the input. But how could you have known that was the problem?

When it doesn't make sense to you, add more debugging!

my $number = <STDIN>;
say ">$number<";
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Hope this helps!

Use chomp:

....
say 'Enter a number: ';
my $number = <STDIN>;
chomp( $number );
say $number;
....
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Produces:

Enter a number: 
4
4
bash
done
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Also check out the function exists. It lets you test if a key is in a hash. In this case it would help you debug but it's useful in general for validating hash keys, especially when they come from user input.

I added the following at the end of your script:

if( exists $j{$number} ) {
    say $j{$number};
    say "done";
}
else {
    say "<$number> is not a key in \%j"
}
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Here is the result of running it:

C:\usr\pm\hash>perl 1214633.pl
4
4

<4
> is not a key in %j
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Thank you all!

Use chomp:

....
say 'Enter a number: ';
my $number = <STDIN>;
chomp( $number );
say $number;
....
[download]

Produces:

Enter a number: 
4
4
bash
done
[download]

#!/usr/bin/env perl

use strict;
use warnings;
use utf8;
use feature 'say';
use File::Copy;

my %j = (
    1  => 'vimplugins',
    2  => "vimwiki",
    3  => "vim",
    4  => "bash",
    5  => "latex",
    6  => "perl",
    7  => "regex",
    8  => "git",
    9  => "gnupg",
    10 => "nmap",
    11 => "tmux",
    12 => "csssnippets",
    13 => "htmlsnippets",
    14 => "latexsnippets",
    15 => "markdownsnippets",
    16 => "perlsnippets",
    17 => "vimsnippets",
);

### PRINT THE SORTED HASH ###
say "$_ - $j{$_}" for sort { $a <=> $b } keys(%j);

### GET USER SELECTION ###
my $number = <STDIN>;
chomp($number);

### PRINT OUT $j{$number} DIRECTLY INSTEAD OF COPYING IT TO ANOTHER BU
+FFER ###
say "\r\b\ryou picked -> $j{$number}\n" if exists $j{$number} or die "
+selection <$number> does not exist in the list!\n";
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