As zwon has pointed out your Structure is not too clear - If it is:

%hash = (
     inst => { 
         dept1 => { 
                 rollno=> { 
                      name  => marks, 
                      name1 => marks1 
                 }, 
                 ...
         },
         ...
      }
      ...
      inst2 ... 
);
[download]

A - rather long, and inefficient solution - but one that is readable and hopefully instructive is below:

use strict;

use Data::Dump qw( dump );

my %hash = (
    inst => { 
    dept => { 
        rollno=> { 
        name  => 100, 
        name1 => 90 , 
        name2 => 30 ,
        name3 => 99 
        }
        
    }
    
    }
    );


my @all_insts = keys %hash;

my @all_deps;
foreach my $single_inst ( @all_insts ) { 

    push @all_deps, keys %{ $hash{ $single_inst } };

}

my @all_roll_nos;
foreach my $single_inst ( @all_insts ) { 

    foreach my $single_dep ( @all_deps ) { 

    push @all_roll_nos, keys %{ $hash{ $single_inst }->{ $single_dep }
+ };

    }

}

my @all_names;
foreach my $single_inst ( @all_insts ) { 

    foreach my $single_dep ( @all_deps ) { 

    foreach my $single_roll_no ( @all_roll_nos ) { 

        push @all_names, keys %{ $hash{ $single_inst }->{ $single_dep 
+}->{ $single_roll_no } };

    }

    }
    
}

    
my @all_marks ;
foreach my $single_inst ( @all_insts ) { 

    foreach my $single_dep ( @all_deps ) { 

    foreach my $single_roll_no ( @all_roll_nos ) { 

        foreach my $single_name ( @all_names ) {
        push @all_marks, $hash{ $single_inst }->{ $single_dep }->{ $si
+ngle_roll_no }->{ $single_name };
        }
        
    }
    
    }
    
}

my @sorted_marks = sort( { $a <=> $b } @all_marks );

dump( @sorted_marks );
[download]

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