Buttons are GUI specific, so unless you tell us what the GUI is you want it to be implemented in, it will be hard to get working examples. perl/Tk button code won't work in Wx and vise-versa.

A callback to dump a table to CSV could be something like (untested!)

# assuming the database handle in $dbh
use Text::CSV_XS;

sub save_table
{
    my $tbl = shift;
    # it is up to you to check for SQL-injection on $tbl ...
    my $sth = $dbh->prepare ("select * from $tbl");
    $sth->execute;
    my %rec;
    my @fld = @{$sth->{NAME_lc}};
    $sth->bind_columns (\@rec{@fld});
    my $csv = Text::CSV_XS->new ({ binary => 1, auto_diag => 1, eol =>
+ "\n", always_quote => 1 });
    $csv->print (*STDOUT, \@fld); # Print header, change STDOUT to fil
+e if required
    while ($sth->fetch) {
        $csv->print (*STDOUT, [ @rec{@fld} ]);
        }
    } # save_table
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