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package Math::Interval;
sub matrix_norm {
    my ($mat_ref) = @_;
...
}
package main;
my $a = Math::Interval::_interval(0.333333, 0.333334);

$ 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.
...
$ 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.