Your example shows that "sub-records" can have odd number of elements and therefore can't be "composed of KEY, VALUE pairs". Obviously, some sub-records are arrays, not hashes. With such loose brief, there's room for interpretation whether to parse sub-record into array or hash. My attempt below assumes "keep arrays for odd number of elements or if unapproved keys were encountered". (E.g. for "us-east-1 us-west-1" sub-record, are they key-value pair or 2-element list?) Obviously, these rules can be adjusted, but idea was to let Perl parse input as Perl source, with only minimal text pre-processing, and always assume arrays. Afterwards, promote some arrays to hashes if they pass rules mentioned above.

use strict;
use warnings;
use Data::Dumper;

my $s = <<'END';
sys ecm cloud-provider /Common/aws-ec2 {
    description "The aws-ec2 parameters"
    property-template {
        account { }
        availability-zone {
            valid-values { a b c d }
        }
        instance-type {
            valid-values { t2.micro t2.small t2.medium }
        }
        region {
            valid-values { us-east-1 us-west-1 }
        }
    }
}
END

my %valid_keys = map { $_, 1 } qw/
    description
    property-template
    account
    availability-zone
    instance-type
    valid-values
    region
/;

$s =~ s/
(?|
    "([^"{}]+)"
    |
    (?:^|(?<=\s))
        ([^\s{}]+)
    (?:$|(?=\s))
)
/"$1"/xg;
$s =~ s/("|})\s\K/,/g;
$s =~ tr/{}/[]/;

print Dumper fix_aref([ eval $s ]);

sub fix_aref {
    my $aref = shift;
    
    ref and $_ = fix_aref($_) for @$aref;
    
    return $aref unless $#$aref % 2;
    
    for ( 0 .. $#$aref ) {
        next if $_ % 2;
        return $aref 
            unless exists $valid_keys{ ${$aref}[$_] }
    }
    return +{ @$aref }
}
[download]

Output:

$VAR1 = [
          'sys',
          'ecm',
          'cloud-provider',
          '/Common/aws-ec2',
          {
            'property-template' => {
                                     'region' => {
                                                   'valid-values' => [
                                                                      
+ 'us-east-1',
                                                                      
+ 'us-west-1'
                                                                     ]
                                                 },
                                     'availability-zone' => {
                                                              'valid-v
+alues' => [
                                                                      
+            'a',
                                                                      
+            'b',
                                                                      
+            'c',
                                                                      
+            'd'
                                                                      
+          ]
                                                            },
                                     'instance-type' => {
                                                          'valid-value
+s' => [
                                                                      
+        't2.micro',
                                                                      
+        't2.small',
                                                                      
+        't2.medium'
                                                                      
+      ]
                                                        },
                                     'account' => {}
                                   },
            'description' => 'The aws-ec2 parameters'
          }
        ];
[download]

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