Your SSCCE was neither self-contained nor correct/compilable.

To be able to help you, I added the use-modules at the top, plus I aliased the CSV bareword-filehandle to the built-in DATA bareword-filehandle, so that reading from <CSV> will actually read from the __DATA__ section at the end of the file:

#!perl

use 5.012; # strict, //
use warnings;
use Excel::Writer::XLSX;
*CSV = *DATA;   # makes CSV an alias to the __DATA__ section, so I can
+ add CSV data to the end of the script, rather than creating a separa
+te file; you will not want this line in your application
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... to the top. And then I created the __DATA__ section at the end of the script, with some CSV-style data:

__DATA__
freq,b xtalk
10,0.000001
20,0.000002
50,0.000005
100,1.00E-05
200,2.00E-05
500,5.00E-05
1000,1.00E-04
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(I only did columns A:B; data in C:E wasn't necessary to prove that the legend and chart will work.)

For your program, you won't need the *CSV = *DATA, nor the __DATA__ section at the end, because presumably you already opened CSV in some line of code not shown to us. (Read perldoc -f open, and learn to use lexical filehandles rather than the old-fashioned bareword filehandles. And don't forget to check for success on your open: it's easiest to do that automatically by using use autodie;)

With the solidfill legend code in there, it gives me the warning

Can't use string ("solidfill") as a HASH ref while "strict refs" in us
+e at c:/usr/local/apps/berrybrew/perls/system/perl/site/lib/Excel/Wri
+ter/XLSX/Chart.pm line 1158.
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and doesn't create the chart at all. If I comment that out, so none of the set_legend calls are active, I get a chart that I think matches your expectation (though the legend is on the center-right and isn't formatted).

I don't know where you got fill=>'solidfill', as I don't see "solidfill" anywhere in the chart formatting section. I tried

$chart->set_legend(
            position => 'overlay_top_right',
            fill => { color => 'red' },
          );
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... and it put it over the chart in the top right with a red background, which is what I'd expect.

You claimed

#$chart->set_legend({ 'font'=> { 'bold' => 1, 'italic' => 1}}); <<== t
+aken from CPAN example, it generates error
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... but the documentation actually says in the set_legend() section,

$chart->set_legend( font => { bold => 1, italic => 1 } );
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... Notice, it only has one pair of braces, not two like in your code. Using exactly the line that the POD shows, I get a legend on the far right whose labels are bold and italic.

Combining it all together

$chart->set_legend(
            position => 'overlay_top_right',
            fill => { color => 'red' },
            font => { 'bold' => 1, 'italic' => 1},
          );
[download]

I get upper right, red background, bold and italic, exactly as I would expect.