# choose an appropriate separator char, I like to use ':'
# using concat() gives us a single column result for a two
# (or more) column need

my $sql_select =
  "select distinct concat(LiftChr, ':', LiftPos) from $tableName";

# since we're requesting a single final column, we can use
# the selectcol_arrayref() to get all values at once

my $array_ref = $db->selectcol_arrayref($sql_select);
# or die?  check for errors!

# now we have all keys and we're not in an active statement either

my $sql_delete =
  "delete from $tableName where LiftChr=? and LiftPos=?";

my $sth1 = $db->prepare($sql_delete);

# this loop steps through a simple array
# waaay faster than fetching row by row

foreach (@$array_ref) {
    # separate the 'single' column back into it's parts
    my ($chr, $pos) = split(/:/, $_);

    # profit
    $sth1->execute($chr, $pos);
}