use Data::Dumper;

my $regex = qr/
              ^field(\d+):       # find a line starting by 'field' and capture its number
              (.*?)\n?           # find the smallest string before the next
              (?=^field\d+:|\z)  # line starting by 'field' or end of record. Rewind just before that point after the match.
            /msx; # ^ matches beginning of line, . matches \n and spaces and comments are ignored in the regex


my %result;
my $count = 1;
{ # block to limit the scope of local
  local $/ = ""; # records are separated by empty lines
  while(<DATA>)
  {
    my %hash;
    while(/$regex/g)
    {
      $hash{"field$1"} = $2;
    }
    $result{"record ".$count++} = \%hash;
  }
}
print Dumper \%result;

__DATA__
field1: data 1 monday
field2: data 2 monday
field3: data 3 monday

field1: data 1 tuesday
field2: data 2 tuesday
        tuesday details line 1
        tuesday details line 2
field3: data 3 tuesday