Read data into a hash-of arrays data structure, then sort it:

use warnings;
use strict;

my %data;
my $key;
while (<DATA>) {
    chomp;
    if (/#(\d+)/) {
        $key = $1;
    }
    else {
        push @{ $data{$key} }, $_;
    }
}

for my $key (sort {$a <=> $b} keys %data) {
    print "#$key\n";
    print "$_\n" for @{ $data{$key} };
    
}
__DATA__
#12346
orange
#12345
apple
banana
#12344
pear
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• perlintro
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• sort