package Math::Interval;
sub matrix_norm {
    my ($mat_ref) = @_;
    my @row_sums;
    foreach my $row_ref (@$mat_ref) {
        my $sum = reduce { $a->abs() + $b->abs() } @$row_ref;
        push @row_sums, $sum;
    }
    return max @row_sums;
}
package main;
my $a = Math::Interval::_interval(0.333333, 0.333334);

##</code><code>##

$ perl -Mstrict -e 'print $a; die'
Name "main::a" used only once: possible typo at -e line 1.
Use of uninitialized value in print at -e line 1.
Died at -e line 1.

$ perl -Mstrict -e 'print $abc; die'    ## die not reached
Global symbol "$abc" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.