# you already have this part:

my $qry_file = "some_name.sql";  # or whatever
open( QRY, ">$qry_file" ) or die "can't write $qry_file: $!";
print QRY "select foo from bar where baz='x'\n";
close QRY;

# Run isql by opening it as a pipeline
# (I'm guessing at the command-line usage):

my @fnames, @lnames;  # query output will be stored here

# update: using the correct variable name for the query file:
open( DB, "isql $qry_file |" ) or die "can't run isql: $!";

while (<DB>) {  # assuming the tool prints one line per row
    chomp; # remove newline
    my ( $lname, $fname ) = split(/\t/); # assuming fields are tab-delimited

    push @fnames, $fname;
    push @lnames, $lname;
}
close DB;

# now you can do whatever you want, iterating over
# the contents of @lnames and/or @fnames

for my $i ( 0..$#fnames ) {
   print "Row = $i, Full_name = $fnames[$i] $lnames[i]\n";
}