#!/usr/bin/perl
use strict;
use warnings;

use DateTime;
use DateTime::Format::Strptime;
use Datetime::Set;

# use Data::Dumper;

my $strp = DateTime::Format::Strptime->new(
                    pattern => '%Y-%m-%d %T',
        );


my $datf = qq{2012-01-01 04:00:00};
my $datt = qq{2012-01-02 23:00:00};
        
my $start = $strp->parse_datetime($datf);
my $end = $strp->parse_datetime($datt);

while( $start < $end ){
    
    print "$start\n" if $start->hour > 6 and $start->hour < 23;
    $start->add( hours => 1 );
}
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$set = DateTime::Set->from_recurrence( 
        after => $start, 
        before => $end,
        recurrence => sub {
        return $_[0]->truncate( to => 'day' )->add( hours => 1 ) 
        }, 
    );
my $iter = $set->iterator;
while ( my $dt = $iter->next ) 
{
        print $dt->ymd;
};
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my $set2 = $set->grep( 
        sub {
            return ( $_->hour > 7 or  $_->hour < 23);
        }
    );    
    
my $iter = $set2->iterator;
while ( my $dt = $iter->next ) 
{
        print $dt->ymd;
};
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The "recurrence" function should return the "next" recurrence. This is one way to implement it:

#!/usr/bin/perl
use strict;
use warnings;

use DateTime;
use DateTime::Format::Strptime;
use Datetime::Set;

# use Data::Dumper;

my $strp = DateTime::Format::Strptime->new(
                    pattern => '%Y-%m-%d %T',
        );


my $datf = qq{2012-01-01 04:00:00};
my $datt = qq{2012-01-02 23:00:00};

my $start = $strp->parse_datetime($datf);
my $end = $strp->parse_datetime($datt);

my $iter;

my $set = DateTime::Set->from_recurrence(
        after => $start,
        before => $end,
        recurrence => sub {
        return $_[0]->add( hours => 1 )->truncate( to => 'hour' )
        },
    );

print "first set:\n";

$iter = $set->iterator;
while ( my $dt = $iter->next )
{
        print $dt->datetime, "\n";
};

my $set2 = $set->grep(
        sub {
            return ( $_->hour > 7 && $_->hour < 23);
        }
    );

print "second set:\n";

$iter = $set2->iterator;
while ( my $dt = $iter->next )
{
        print $dt->datetime, "\n";
};
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Thank you very much fglock, also for the comment on Re: A question to iterator in Datetime::Event::Recurrence !