As a demo, I re-wrote your a1 sub and added an a2 sub.

Rather than assigning the hash reference in the subroutine to the "global" variable, I would recommend returning the hash reference from the sub as shown in recoded sub a1.

Like in C, it is possible to pass a "pointer", the hash reference to a sub as shown in sub a2. The memory previously used by sub a1 is "re-used", same struct is modified. If "my $href" goes out of lexical scope, its memory will be recovered and reused by Perl.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;    # a cool core module
                     # that dumps any structure

my $href = a1();  #hash reference returned from sub
print "v1=$href->{'v1'}\n"; #dot operator not needed

print Dumper $href;

a2($href);
print Dumper $href;

exit(0);

sub a1
{
   my %hash;
   
   $hash{v1} = 10;
   $hash{v2} = 20;
   
   return \%hash;  # %hash memory will "live" due to
                   # reference counting
}

sub a2
{
   my ($href) = @_;
   
   $href -> {v1} = 30;  #quotes are ok but not needed
   $href -> {'v2'} = 40;
   return;
   
}

__END__
v1=10
$VAR1 = {
          'v2' => 20,
          'v1' => 10
        };
$VAR1 = {
          'v2' => 40,
          'v1' => 30
        };
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