If you search for a single column and your query is guaranteed to return at most one row, you can use selectrow_array to do everything at once:

my $sql = "SELECT username FROM users WHERE username = ?";
my $user = $dbh->selectrow_array( $sql,{},$Username_POST );
if( $user ) { die "User '$user' exists; }
else { ... # user doesn't exist }
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Note: I dropped the semicolon at the end of your SQL - you don't want that there. I also used placeholders, which is a better way to go in general.

Update Or, if you want the whole row:

my $sql = "SELECT username FROM users WHERE username = ?";
my @row = $dbh->selectrow_array( $sql,{},$Username_POST );
if( @row ) { die "User exists"; }
else { ... # user doesn't exist }
[download]