The misunderstanding may be regarding what happens when you assign a hash ref to a glob rather than what local is doing.

#!/usr/bin/perl 

use strict;
use warnings;
use Data::Dumper;


sub fillerup ($);

my %fillme = (gas_pr => 3.59, gas_capacity => 14);


local $\ = "\n";

print '-'x80;
foreach my $key (keys %fillme  ) {
    print $key . '=' . $fillme{$key};
}

fillerup ( \%fillme );

print '-'x80;

foreach my $key (keys %fillme  ) {
    print $key . '=' . $fillme{$key};
}

sub fillerup ($) {
    no strict; 
    local *hashref = shift;
#    my $hashref = shift;

        print "Prior to inserting a record in fillerup...";
    print '*'x80;

    foreach my $key (keys %$hashref  ) {
        print $key . '=' . $hashref->{$key};
    }

    $hashref->{filled_up} = 12.3;
    
    print '*'x80;

    foreach my $key (keys %$hashref  ) {
        print $key . '=' . $hashref->{$key};
    }

    print '*'x80;
    print "\*hashref:   " . Dumper(*hashref);
    print "\$hashref:   " . Dumper($hashref);
    print "\%\$hashref:  " . Dumper(%$hashref);
    print "\\\%\$hashref: " . Dumper(\%$hashref);
    print "\\\%hashref:  " . Dumper(\%hashref);
    print "\\\%fillme:  " . Dumper(\%fillme);

    return;
}
[download]

Produces

----------------------------------------------------------------------
+----------
gas_capacity=14
gas_pr=3.59
Prior to inserting a record in fillerup...
**********************************************************************
+**********
**********************************************************************
+**********
filled_up=12.3
**********************************************************************
+**********
*hashref:   $VAR1 = *::hashref;

$hashref:   $VAR1 = {
          'filled_up' => '12.3'
        };

%$hashref:  $VAR1 = 'filled_up';
$VAR2 = '12.3';

\%$hashref: $VAR1 = {
          'filled_up' => '12.3'
        };

\%hashref:  $VAR1 = {
          'gas_capacity' => 14,
          'gas_pr' => '3.59'
        };

\%fillme:  $VAR1 = {
          'gas_capacity' => 14,
          'gas_pr' => '3.59'
        };

----------------------------------------------------------------------
+----------
gas_capacity=14
gas_pr=3.59
[download]

Update: See Typeglobs and Filehandles for more info on assignment to typeglobs. Note that assigning the reference to your global hash to your localized typeglob sets %hashref to be an alias to %fillme and does not set $hashref to a reference to the hash as you are expecting. You then autovivify $hashref with a reference to a new hash containing only the key 'filled_up', and leaving %hashref unchanged.

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