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use strict;
use warnings;

$ 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 $input"?) at pm_11103133.pl line 5.
Global symbol "$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.

#!env perl
use strict;
...
    my @largest_number = sort {$a <=> $b} @$input;
    print "$lines\n";
}

$ perl pm_11103133.pl <xy.data
Global symbol "$input" requires explicit package name (did you forget 
+to declare "my $input"?) at pm_11103133.pl line 7.
Execution of pm_11103133.pl aborted due to compilation errors.

$ perl pm_11103133.pl <xy.data
Hello, I'm 18
...
1 this year is 2019 1

1 2 3 - 4

#!env perl
use strict;
...
        $largest_number_so_far = $largest_number[0];
    }
}

$ perl pm_11103133.pl <xy.data
Hello, I'm 18

Use of uninitialized value in numeric gt (>) at pm_11103133.pl line 13
+, <STDIN> line 4.

#!env perl
use strict;
...
        $largest_number_so_far = $largest_number[0];
    }
}

$ perl pm_11103133.pl <xy.data
CURRENT LINE: <Hello, I'm 18
...
Use of uninitialized value $largest_number[0] in concatenation (.) or 
+string at pm_11103133.pl line 14, <STDIN> line 4.
LARGEST IS:
Use of uninitialized value in numeric gt (>) at pm_11103133.pl line 16
+, <STDIN> line 4.

$ cat pm_11103133.pl
#!env perl
...
        }
    }
}

$ perl pm_11103133.pl <xy.data
CURRENT LINE: <Hello, I'm 18
...
NUMBERS FOUND IN LINE:
Use of uninitialized value $largest_number[0] in concatenation (.) or 
+string at pm_11103133.pl line 14, <STDIN> line 4.
LARGEST IS:

$ perl pm_11103133.pl <xy.data
Hello, I'm 18

1 this year is 2019 1

$ cat pm_11103133.pl
#!env perl
...
}

print "$line_with_largest_number\n";

$ perl pm_11103133.pl <xy.data
1 this year is 2019 1