In addition to Corion's response, I would mention that if you DO you want to give $sth->execute() parameters, they need to be question marks in your query itself.. like

my $variable = "E";
my $sql = 'SELECT crcp_cod_grupo, crcp_cod_prato FROM REST.dbo.CRCP_PR
ATO WHERE crcp_cod_grupo = ?';
my $sth = $dbh->prepare($sql);
$sth->execute($variable);
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You would then list each variable in the order you have them as question marks in your query
Also, I normally put the query in double quotes instead of single quotes..
UPDATE: Are you trying to execute only the rows that you are incorrectly passing as parameters to execute?