Update:

roboticus, You have assumed that the input is a list of words but it is of the format:

Word Frequency
Word Frequency
Word Frequency
Word Frequency
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Given this, the code should probably be:

my %H;
while (<>) {
    my ( $word, $freq ) = split /\s+/, $_;
    $word = lc( $word ) ;
    $H{ $word } += $freq ;
}
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Although that does not change your primary thesis: people frequently miss the opportunity to clean up the data before storing it

The way to achieve this with minimum changes to your code is in my original response:

Original Response:

I think you meant:

$H{ $words[0] } += $words[1] ;
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