$str = 'first name       EOFlast name        EOFaddress field              EOF';

my @rec_def = (
    [ 'first_name', 20 ],
    [ 'last_name',   20 ],
    [ 'address',    30 ],
);

sub parse_fixed_width {
    my ( $record, $rec_def ) = @_;
    my %struct;
    my $offset = 0;
    for my $rec(@$rec_def) {
        $struct{$rec->[0]} = substr $record, $offset, $rec->[1];
        $offset += $rec->[1];
    }
  return length($record) == $offset ? \%struct : '';
}

use Data::Dumper;
print Dumper parse_fixed_width( $str, \@rec_def );
__DATA__
$VAR1 = {
          'first_name' => 'first name       EOF',
          'address' => 'address field              EOF',
          'last_name' => 'last name        EOF'
        };