I believe LEFT() ought to take two parameters; one being a string to extract characters from and two being the number of characters to extract. Maybe you should try something like

my $l = length($var);
my $query = "SELECT field1,field2 FROM my_table WHERE LEFT(field3,$l) 
+= '$var'";
[download]

---
print map { my ($m)=1<<hex($_)&11?' ':'';
$m.=substr('AHJPacehklnorstu',hex($_),1) }
split //,'2fde0abe76c36c914586c';
[download]