If the line contains only the string apple, and you split it on comma (line 16), the newline at the end of the line stays in $line[0]. Therefore, the hash doesn't return anything for the key. Use chomp to remove the newline:

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

my %fruit;

open my $FRUIT, '<', 'fruit.csv' or die $!;
while (<$FRUIT>) {
    my @line = split /,/;
    my($fruit_name, $number) = @line;
    $fruit{$fruit_name} = $number;
}
close $FRUIT;


open my $FRUITNAMES, '<', 'fruit_names.csv' or die $!;
while (<$FRUITNAMES>) {
    chomp;
    my @line = split /,/;
    my $thefruitsname = $line[0];
    warn ": $thefruitsname";
    my $thenumber = $fruit{$thefruitsname};
    print "the number of $thefruitsname is $thenumber\n";
}
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Also use indentation, 3 argument open + or die, and strict as shown in the script.