use warnings;
use strict;

my %arith;
$arith{"+"} = sub { $_[0] + $_[1] };
$arith{"-"} = sub { $_[0] - $_[1] };
$arith{"*"} = sub { $_[0] * $_[1] };
$arith{"/"} = sub { $_[0] / $_[1] }; 

my $problem = "6 + 1";
my $answer;
if ( 
    $problem =~ /^(\d+)\s*(\S)\s*(\d+)$/ 
    and exists $arith{$2} 
) {
    eval {
        $answer = $arith{$2}->( $1, $3 );
    }
} else {
    die "Oops!\nOperation syntax not understood.\n";
}
die "Woops!\n$@" if $@;
print "$answer\n";