use strict;
use warnings;

use CGI;

my $cgi = CGI->new();

sub handle_error {
    print $cgi->header(
        -status => 500,
        -type   => 'text/html',
    );
    print ...;
    exit(0);
}

open(my $fh, '<', 'foo.csv')
    or handle error(...);

print $cgi->header(
    # application/octet-stream might work better
    -type       => 'text/csv',
    -attachment => 'foo.csv',
);

print while <$fh>;