You're passing a single value, \@temp, but you appear to want to pass @temp instead

#!/usr/bin/perl --
use strict;
use warnings;
my @temp = ( 4, 8, 3 );
my $quadratic = quadratic (\@temp);
print "$quadratic\n";

$quadratic = quadratic (@temp);
print "$quadratic\n";

sub quadratic {
    my ($a, $b, $c) = @_;
    return (-$b + sqrt($b*$b - 4*$a * $c)) / 2*$a;
}
__END__
Use of uninitialized value $b in negation (-) at - line 13.
Use of uninitialized value $b in multiplication (*) at - line 13.
Use of uninitialized value $b in multiplication (*) at - line 13.
Use of uninitialized value $c in multiplication (*) at - line 13.
0
-8
[download]