use strict;                             # https://perlmonks.org/?node_id=11136806
use warnings;                           # creating the hierarchy pattern 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 => {} }," etc

                                        # 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