use strict;
use warnings;

use Data::Dumper;

my %levels = ();
my $level  = 0;

while (<DATA>) 
{
    chomp;
    
    if (/^level (\d+)/) 
    {
        $level = $1;
    }
    elsif (/^section(\d+)/) 
    {
        push @{$levels{$level}}, $_;
    }
}

print Dumper (\%levels);
__DATA__
level 1
section1
section2
level 2
section3
section4
level 2
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Output:

  
$VAR1 = {
          '1' => [
                   'section1',
                   'section2'
                 ],
          '2' => [
                   'section3',
                   'section4'
                 ]
        };
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