... By structure of data I mean which variable is in which place and its name/type.

Maybe specify a template similar in spirit to Perl's pack, e.g. something like

    "a,a,c,i,a,c,..."
    
    # to be used in:
    my ($foo, $bar, $baz, ...) = unpack_line("a,a,c,i,a,c,...", $line)
+;
    # or
    my $parser = My::Module->new( template => "a,a,c,i,a,c,..." );
    my ($foo, $bar, $baz, ...) = $parser->read_line();
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(where a / c / i would stand for categorical / continuous / identifier, etc.)

Or together with names:

    "foo:a, bar:c, baz:i, ..."

    my %fields = unpack_line("foo:a, bar:c, baz:i, ...", $line);
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Of course, there are many other ways to approach this, so possibly ask your potential users what interface or mini template language they would prefer...