$ perl pm_11103133.pl <xy.data
Global symbol "$lines" requires explicit package name (did you forget to declare "my $lines"?) at pm_11103133.pl line 3.
Global symbol "@input" requires explicit package name (did you forget to declare "my @input"?) at pm_11103133.pl line 4.
Global symbol "$lines" requires explicit package name (did you forget to declare "my $lines"?) at pm_11103133.pl line 4.
Global symbol "@largest_number" requires explicit package name (did you forget to declare "my @largest_number"?) at pm_11103133.pl line 5.
Global symbol "$input" requires explicit package name (did you forget to declare "my $input"?) at pm_11103133.pl line 5.
Global symbol "$lines" requires explicit package name (did you forget to declare "my $lines"?) at pm_11103133.pl line 6.
Execution of pm_11103133.pl aborted due to compilation errors.