I've refactored your code slightly and I hope you can see how the act loop is setting the same hash element each time. Add the $debug variable to see. What structure do you want ?

foreach my $i ( 1 .. $scheduled_export_days ){

  my %column_data_export;
  my %column_data_import;
  $column_data_export{SCHEDULE} = $schedule_hour;
  $column_data_export{UNDEF}    = 1;

  my $debug=0;
  for my $act (@{$bi->{$customer}->{$domain}->{$host}->{BACKUPDATA}}){
+    
    ++$debug;
    my $imp_exp_type   = $act->{ACTION_TYPE};
    my $imp_exp_status = $act->{STATUS};
    my @schedule_time  = split(/:/, $schedule_hour);
    my $minutes_range = ($schedule_time[1]+30) % 60;    

    # import code
    if ($imp_exp_type eq "IMPORT"){

      my ($imp_formated_hour,undef) = hhmm($act->{ENDDATE_SEC},$unit)
      $column_data_import{SCHEDULE} = $imp_formated_hour;
      $column_data_import{UNDEF}    = $debug ;#1 ;

    }    
    
    # export code
    
  }
  push (@columns_export, \%column_data_export);        
  push (@columns_import, \%column_data_import);
}  

sub hhmm {
  my ($s,$unit) = @_;
  my @tmp = split / /,$unit->formatDateTime($s);
  my ($hr,$min) = split /:/, $tmp[1];    
  return ("$hr:$min",$tmp[0]);
}
[download]

my wished structure is like the dumper result in my original post. As you can see there, the SCHEDULE_COLUMNS_IMPORT has several entries. The schedule value here should display the value of $imp_formated_hour from all the existing export reports. So it should basically be the same as the SCHEDULE_COLUMNS_EXPORT, just with other schedule_values. The difference here is, the schedule value for the export reports are gathered in a loop before. Not like the schedule values of the import reports. I don't know how I can change my code so that it doesn't overwrite the hash all the time .. sorry, my PERL knowledge is still very lacking :/

Are the import times held like this and if so how do they relate to variable $scheduled_export_days ?.

#!perl
use strict;
use Data::Dump 'pp';

my $bi->{customer}{domain}{host}{BACKUPDATA}=[
  { ENDDATE_SEC => 1 },
  { ENDDATE_SEC => 2 },
  { ENDDATE_SEC => 3 },
  { ENDDATE_SEC => 4 },
  { ENDDATE_SEC => 5 },
];
pp $bi;

for my $act (@{$bi->{customer}->{domain}->{host}->{BACKUPDATA}}){    
  print $act->{ENDDATE_SEC}."\n";
}
[download]

poj