The probable reason is that you are storing numeric values as PVs--their string representation as read from the file--in addition to IVs--their numeric representation--the generation of which you are deliberately forcing with this code:

if ($self->datatype->[$_] =~ 
    /^(?:TINYINT|MEDIUMINT|SMALLINT|INT|INTEGER|BIGINT|FLOAT|DOUBLE)$/
+) {
    $hr_returnvalue->{$CurrentColumnName} *= 1;
                      # Multiply by 1 to create a numeric value.
}
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Having initially loaded the value as a string (PV), when you force it to be converted to a numeric value (IV), the string value will be retained so that if you later decide to use it in a string context, the (inverse) conversion does not have to be repeated.

Eg. After the *= 1;, the PV is still there, but you gained an IV. Essentially you've increased the size of the SV rather than reduce it (as I assume you intended):

C:\test>perl -MDevel::Peek -E"my $x = '12345'; Dump $x; $x *=1; Dump $
+x"
SV = PV(0x6cf50) at 0xc74e8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x67758 "12345"\0
  CUR = 5
  LEN = 8

SV = PVIV(0xaf018) at 0xc74e8
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 12345
  PV = 0x67758 "12345"\0
  CUR = 5
  LEN = 8
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A fix would be to perform the string->numeric conversion before storing the value:

for( 0 .. $#{$self->columns} ) {
    $CurrentColumnName = $self->columns->[$_];
    if( !(defined $self->flatfield_start->[$_] 
    and defined $self->flatfield_length->[$_] ) 
    ) {
    # Field is missing interface_start, interface_length or both, skip
+ it.
    next;
    }
    if ($self->datatype->[$_] =~
         /^(?:TINYINT|MEDIUMINT|SMALLINT|INT|INTEGER|BIGINT|FLOAT|DOUB
+LE)$/
    ) {
        $hr_returnvalue->{$CurrentColumnName} = 0 + 
            substr( $textinput, 
               $self->flatfield_start->[$_], $self->flatfield_length->
+[$_] );
    }else {
        $hr_returnvalue->{$CurrentColumnName} = 
             substr( $textinput, 
                 $self->flatfield_start->[$_], $self->flatfield_length
+->[$_] );
            $hr_returnvalue->{ $CurrentColumnName } =~ 
                 s/^\s*(.*?)\s*$/$1/;    # Trim whitespace
    }
        
 # Fill empty fields with that field's default value, if such a value 
+is defined
[download]

That should reduce the size of the final data structure significantly if there are many numeric fields.