Could you post the code where this sub is called?

Including the declaration & initialisation of the $compound input parameter.

Here is everything, there is a little that isn't directly related, but cutting it out won't improve readability much and may be important so I left it. $compound is created in &find_symmetry_file which is the last sub in this script but it is used throughout. Thanks.

#!/usr/bin/perl
use Data::Alias;
use strict;
use warnings;
use threads;
use Thread::Queue; 

### Set parameters for this run, no. of threads, where to print, what 
+to read
my $thread_count = shift @ARGV;
my $output_dir = shift @ARGV;
our $THREADS = $thread_count;
my $file_q = Thread::Queue->new();
my $result_q = Thread::Queue->new();

###Make threads enqueue work
my @threads = map { threads->create( \&thread_wrapper, $file_q, $resul
+t_q ); } 1..$THREADS;
while ( @ARGV )
  { 
  my $file = shift @ARGV;
  $file_q->enqueue($file);  
  }
sleep 1;


###Threads done, join them and wait
$file_q->enqueue((undef) x $THREADS);
map {$_->join();} @threads;
sleep 1;

###Print results
use JSON;
if (!(-d "$output_dir"))
  {
  mkdir $output_dir;
  }
for (1..$THREADS)
  {
  while ( my $result = $result_q->dequeue )
    {
    my $json_result = encode_json $result;
#    print $json_result, "\n";
    my $target = $output_dir . $$result{ID} . "_entry". $$result{Entry
+} . ".txt.DONE";
    print $json_result, "\n";
#    open(RESULT, '>', $target);
#    print RESULT $json_result;
#    close RESULT;
    }
  }

###Subs

###This is what the threads need to do, not very elegant but easy to m
+ake from the single thread version

sub thread_wrapper
  {
  my ( $tid, $Qwork, $Qresults ) = ( threads->tid, $_[0], $_[1] );
  while ( my $work = $Qwork->dequeue )
    {
    my $compound = &find_symmetry_file("$work");
    $Qresults->enqueue($compound);
    }
  $Qresults->enqueue(undef);
  }


###Find all mapping vectors for the molecule

sub find_symmetry
  {
  #use Data::Alias; #this gives same errors.
  my $compound = shift @_;
  $$compound{Symmetries} = [];
  my $M_matrix = $$compound{Mapping};
  my $A_matrix = $$compound{Adjacency};
  my @isomorphs;
  my $row = 0;
  my $dimension = $#$A_matrix;
  my @indices = (0..$dimension);
  my @reverse = reverse @indices;
  my @used = map {0;} (@indices);
  my @mapping = map {-1;} (@indices);
  my (@sm_rows, @same_rows);
  foreach my $row (@$A_matrix) { push @sm_rows, $row; }
  foreach my $row (@$M_matrix) { push @same_rows, $row;}
  my $equate_sub = sub 
    {
    for (reverse(0..$row))
      {
      my (@smrow, @omrow);
      alias @smrow = @$sm_rows[$_];
      alias @omrow = @$sm_rows[$mapping[$_]];
      for (0..$row)
        {
        if ($smrow[$_] != $omrow[$mapping[$_]])
          {
          return 0;
          }
        }
      }
    return 1;
    };
  LOOP: while(1)
    {
    do { $mapping[$row] ++ } while ( $mapping[$row] <= $dimension and 
+($used[$mapping[$row]] == 1 || $same_rows[$row][$mapping[$row]] == 0 
+));
    next if ($mapping[$row] > $dimension);
    if ( $row == $dimension )
      {
      OUTER:
        {
        for (@indices)
          {
          my (@smrow, @omrow);
          alias @smrow = @$sm_rows[$_];
          alias @omrow = @$sm_rows[$mapping[$_]];
          for (@reverse)
            {
            if ( $smrow2[$_] != $omrow2[$mapping[$_]] )
              {
              last OUTER;
              }
            }
          
          }
        push @{$$compound{Symmetries}}, [@mapping];
        }
      }
    elsif ( $row == 0 or &$equate_sub == 1 )
      {
      $used[$mapping[$row]] = 1;
      $row++;
      }
    }
  continue
    {
    while ( $row >= 0 and $mapping[$row] >= $dimension )
      {
      last LOOP if ($row == 0);
      $mapping[$row] = -1;
      $row --;
      $used[$mapping[$row]] = 0;
      }
    }
  delete $$compound{Mapping}; #don't need this anymore;
  delete $$compound{Ajdacency}; #don't need this anymore;
#  $$compound{Mapping} = undef; #same as above but leaves key so JSON 
+ain't pretty
#  $$compound{Adjacency} = undef;
#  return \@isomorphs;
  }


###Make M_matrix for molecule
sub make_mapping_matrix
  { 
  my $compound = shift @_;
  my @types = @{$$compound{KEGG_Types}};
  my $mapping = $$compound{Mapping};
  for my $i (0..$#types)
    {
    for my $j ($i..$#types)
      {
      if ($types[$i] eq $types[$j])
        {
        $$mapping[$i][$j] = 1;
        $$mapping[$j][$i] = 1;
        }
      }
    }
  delete $$compound{KEGG_Types};
#  $$compound{KEGG_Types} = undef;   
  }

###Make A_matrix for molecule
sub make_adjacency_matrix
  {
  my $compound = shift @_;
  my @bonds = @{$$compound{Bonds}};
  my $adjacency = $$compound{Adjacency};
  foreach my $bond (@bonds)
    {
    $$adjacency[$$bond[0]-1][$$bond[1]-1] = $$bond[2];
    $$adjacency[$$bond[1]-1][$$bond[0]-1] = $$bond[2];
    }
  delete $$compound{Bonds};
#  $$compound{Bonds} = undef;
  }


###parse kcf file, make compound representation, make matrices, find s
+ymmetry and return.

sub find_symmetry_file
  {
  my $file = shift @_;
  open FILE, $file;
  my %compound; 
  my $parser_state = 0;
  OUTER: while ( my $line = <FILE> )
    {
    chomp $line;
    if ( $parser_state == 1 and $line =~ /COMPOUND/)
      {
      $parser_state++;
      $compound{"ID"} = join("",split(/COMPOUND|\s/,$line));
      }
    elsif ( $parser_state == 0 and $line =~ /^1|^2/ ) 
      {
      $parser_state++; 
      $compound{"Entry"} = $line;
      }
    elsif ( $parser_state == 2 and $line =~ /ATOM/ )
      {
      $parser_state++;

#      $compound{"Atom_Count"} = join("",split(/ATOM|\s/,$line));
      my $atom_count = join("",split(/ATOM|\s/,$line));
      $compound{"KEGG_Types"} = [];
      $compound{"Mapping"}  = [ map { [ map { 0; } (0..$atom_count-1) 
+]; } (0..$atom_count-1) ];
      $compound{"Adjacency"} = [ map { [ map { 0; } (0..$atom_count-1)
+ ]; } (0..$atom_count-1) ]; 
      }
    elsif ( $parser_state == 3 and $line =~ /BOND/ )
      {
      $parser_state++; 
#      $compound{"Bond_Count"} = join("",split(/BOND|\s/,$line));
      $compound{"Bonds"} = [];
      }
    elsif ( $parser_state == 4 and $line =~ /BRACKET/ )
      {
      last OUTER;
      $parser_state++;
      }
    elsif ( $parser_state == 3 ) #and  $line =~ /\s*\d*[A-Z]\d?[a-z]?\
+s*[A-Z]/ )
      {
      my ($junk, $index, $kegg_type, $element_type, $x, $y ) = split(/
+\s+/,$line);
      push @{$compound{"KEGG_Types"}}, $kegg_type;
      }
    elsif ( $parser_state == 4 ) #and $line =~ /\s+\d+\s+\d+\s+/ )
      {
      my ($bond, $junk1)  = split(/#/,$line);
      my ($junk2, $index, $atom_1, $atom_2, $order) = split(/\s+/,$lin
+e);
      my @array = ($atom_1, $atom_2, $order);
      push @{$compound{"Bonds"}}, \@array;
      } 
    }
  &make_mapping_matrix(\%compound);
  &make_adjacency_matrix(\%compound);
  &find_symmetry(\%compound);
  return \%compound;
  close FILE;
  }
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