Just for fun:

use strict;                             # https://perlmonks.org/?node_
+id=11136806
use warnings;                           # creating the hierarchy patte
+rn from the input file
use Data::Dump qw/pp dd/;


my $tree ={};
while ( my $line =<DATA> ) {
    chomp $line;
    my @keys = split /@/, $line;
    my $ref = $tree;
    $ref = ( $ref->{$_} //= {} )        # descend into hashrefs, init 
+explicitly when missing
      for @keys
}

my $out = pp $tree;                     # now "d1 => { d1 => {} }," et
+c

                                        # purge 
$out =~ s/\Q => {}//g;                  # ... empty hashes
$out =~ s/\Q => /    /g;                # ... fat arrows
$out =~ s/\Q},/}/g;                     # ... trailing commas  
print $out;




__DATA__
instreg@d1@d1
instreg@d1@d2
instreg@d2@d1
instreg@d3@d1
instreg@d4@d1
instreg@d5@d1
instreg@d6@d1
instreg@d7@d1
instreg@d8@d1
alureg@d1@d1
alureg@d2@d1
alureg@d3@d1
alureg@d4@d1
alureg@d5@d1
alureg@d6@d1
alureg@d7@d1
alureg@d8@d1
pgmctr@d1@d1
pgmctr@d2@d1
pgmctr@d3@d1
pgmctr@d4@d1
pgmctr@d5@d1
pgmctr@m1
pgmctr@m2
pgmctr@m3
pgmctr@m4
pgmctr@m5
[download]

OUTPUT:

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Thu Sep 16 13:39:54

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/text_to_tree.pl 
{
  alureg     {
               d1    { d1 }
               d2    { d1 }
               d3    { d1 }
               d4    { d1 }
               d5    { d1 }
               d6    { d1 }
               d7    { d1 }
               d8    { d1 }
             }
  instreg    {
               d1    { d1, d2 }
               d2    { d1 }
               d3    { d1 }
               d4    { d1 }
               d5    { d1 }
               d6    { d1 }
               d7    { d1 }
               d8    { d1 }
             }
  pgmctr     {
               d1    { d1 }
               d2    { d1 }
               d3    { d1 }
               d4    { d1 }
               d5    { d1 }
               m1,
               m2,
               m3,
               m4,
               m5,
             }
}
Compilation finished at Thu Sep 16 13:39:55
[download]

alternatively

use Data::Dumper (which is core) and change the settings for => ("Pair") and so on. See https://metacpan.org/pod/Data%3A%3ADumper#Configuration-Variables-or-Methods

update

added more comments to code

---

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