What I might do (but using your actual list of fields and helpful/descriptive names for each field) using Parse::FixedLength:

use Parse::FixedLength;

my $pfl = Parse::FixedLength->new([qw(
  name:10
  date:6
  filler:10
)]);

my %hash = (
  name => 'Joe',
  date => '020414',
);

my $str = $pfl->pack(\%hash);
print "[$str]\n";

$hash{name} = 'Bob';
$str = $pfl->pack(\%hash);
print "[$str]\n";

# Or even:
use Parse::FixedLength;

my $pfl = Parse::FixedLength->new([qw(
  name:10
  date:6
  filler:10
)], {href => \my %hash} );

%hash = (
  name => 'Joe',
  date => '020414',
);

my $str = $pfl->pack();
print "[$str]\n";

$hash{name} = 'Bob';
$str = $pfl->pack();
print "[$str]\n";
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