#!/usr/bin/perl
use strict;
use warnings;
use Carp;

# Table containing numerical grade thresholds
my %grade_for = (
     '90'   => 'A',
     '80'   => 'B',
     '70'   => 'C',
     '60'   => 'D',
     '50'   => 'F',
);

# Determine whether you want to round up or 'chop off' grades
sub _round_grade {
    my $grade = shift;

    # We deal with thresholds in multiples of ten
    return 10 * sprintf( "%02.0f", $grade / 10 );
}

sub _chop_grade {
    my $grade = shift;

    # We deal with thresholds in multiples of ten
    return 10 * int ( $grade / 10 );
}

sub _convert_grade {
    my $grade = shift;

    # Check whether the grade entered is listed in our table
    if ( defined( $grade_for{$grade} ) ) {
        return $grade_for{$grade};
    }
    else {
        # No character representation available; warn the user
        carp "Character grade for $grade not listed in our table.";
        return q{};
    }
}

LINE:
while ( my $line = <STDIN> ) {
    # Strip the newlines from our input
    chomp $line;

    # Prepare a RE to match the line against
    my $grade_re = qr{
        \A          # Start of the string
        \s*         # Allow for leading spacing
        (\d{1,2})   # Match at least 1 digit and at most 2 digits
        \s*         # Allow for trailing spacing
        \z          # End of the string
    }xms;

    # Skip lines that are not a number
    next LINE if $line !~ m{$grade_re}xms;

    # Be strict when dealing out grades; round down our numerical input
    #   $1 is valid, given the previous RE match
    my $grade_numeric = _chop_grade($line);

    # Convert our numeric input to a character
    my $grade = _convert_grade($grade_numeric);

    print "$line - $grade\n";
}