You could try out the study function. However, you'll have to refactor your code so that the outer loop is done on the texts, and of course pull the regex compiling out of there.

Assuming that %hash1 and 2 are different from $hash1_ref, this could become something like (untested):

sub slow_match 
{
    my ( $hash_ref_1, $hash_ref_2 ) = @_;
    # %texts, %patterns coming from global (was: %hash1, %hash2)

    my %matches;
    foreach my $pattern ( keys %$patterns_ref )
    {
        $matches{$pattern} = qr/\b$pattern\b/;
    }

    while ( my ( $text_id, $text ) = each %texts )
    {
        study $text;

        while ( my ( $pattern, $high_lvl_id ) = each %patterns )
        {
            if ( $text =~ $matches{$pattern} )
            {
                $$hash_ref_1{$text_id} .= ':'.$high_lvl_id;
                foreach my $part (split(/\s/,$pattern))
                {
                    $$hash_ref_2{$text_id} -> {$part} = 0;
                }
            }
        }
    }
}
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